I figured, while I still have all of this knowledge, that I should share it before it starts to fade.
This, my friends, is a comprehensive summary of Helsinki nightlife. Should you ever come to Helsinki (with or without me), this is what you could use to guide your decisions for your nighttime activities.
Clubs (in no particular order)
A wonderfully odd electronica bar and club that plays indie DJs. The quality of music is pretty good but sometimes it can be a little hit or miss. Age group tends to be the mid 20's and older, the drinks reasonably priced (about 8euros and higher, depending on what you're getting). Entrance is 10euros which includes coatcheck. There is a separate bar and dance floor (they're separate rooms), though there is also a bar at the dance floor. Dark atmosphere with excellent light effects. Generally very fun.
One of the better places for top 40 hits for music but very hit or miss depending on the deejay's choice (and deejay is spelled out because it's just a compilation of songs that are already made - no mixing whatsoever is involved and there really are pauses between songs sometimes...awkwardly). Age group tends to be mixed between low 20's and older. No entrance fee to get in but coat check is 2,50euros. There are two bars - one as you go in and another at the back of the dance floor. There are poles on two tables should you feel like pole dancing. Fantastically fun atmosphere and the largest mojito list I've ever seen. Normal drinks (salmiakki, minttu, etc) are reasonable, but things of higher quality (shots of Patron, etc) can be pricey (10euros per shot).
One of the few clubs where you can go and watch a live band play top 40 covers. It's a bit hit or miss though - you can either get a great cover band that plays everything you know (usually these are the Swedish bands), or you can walk in on a night where they only play Finnish rock (or Finnish music in general). I've been generally lucky and heard some great renditions of Kings of Leon and Muse. Beer and cider is pretty cheap, don't remember how much mixed drinks are. There is a cover (which is under 10euros I believe) but coat check isn't free. Crowd tends to be mid 20's and older. Big dance floor in front of the stage so you can see the live band no matter where you are.
You go to Namu when you want to dress up in your clubbing clothes and meet other people who look the same (some do better than others in the fashion department). There is an indoor and outdoor dance area (especially when the weather is good) that play music and there are several bars (three at least). There is cover (which I believe is 10euros) though coatcheck is included. The drinks here are expensive. There are no two ways about it. Hungarian Z made the mistake of ordering a long island iced tea and paid 25euros for it - they charge you by each shot of alcohol in your drink, and we all know how many types of alcohol are in a LIIT (too many). General crowd tends to be douchey, early to late 20's, but can be fun if you're looking to dance to slightly older top 40 club hits and want to dress up (most clubs are pretty casual).
Once again one of the few places where you can dress up in your clubbing clothes and get it on. Smaller dance floor than Namu but much classier clientele (as in older, late 20's and beyond, and with more money) and better environs (read: gold filigree and Bellagio-style atmosphere), generally speaking. There is a 7euro cover to get in, but this can be waived if you know the right friend who is a VIP (this is generally true for any club, but this one requires your friend to actually be a member of their VIP club...like with a card). Drinks here are of the cocktail price range (12euros or so) but that's pretty standard given the atmosphere (classy, or tries to be). Music tends to be top 40 club hits and that's great but the absolute d-baggery of the clientele (and the extreme golddigger women who tend to follow them around) can get tiring.
Only stopped in here once for a post-wedding drink, but got a decent impression. Entrance was less than 10euros and the atmosphere was pretty nice (very modern with color changing LEDs installed into everything, leading to a rather confusing but interesting atmosphere). Though the dance floor wasn't happening when we were there I've heard it's quite the club when it is. Music seemed to be some sort of mainstream electronica. Drinks were cocktail standard (12euros or so). Crowd is definitely the mid 20's and older crowd.
Roska is as cheap and dirty while still being fun as you can get, and I mean that in the nicest way possible. Entrance is 3euros and drinks are cheap (like around the 6euro mark). The music is fantastic though (danceable, usually some remixes of recognizable top 40 and other great indie music) so despite the rather desperately cheap digs (imagine some graffitied abandoned basement and you'll get the idea) it's actually acceptable. The crowd here is extremely young (we're talking late teens to early 20's, at the oldest, minus my friends and I), and it shows. There are benches along the side of the dance floor and a lot of incredibly drunk teenagers are usually sitting all over each other by the end of the night. Classy business. Kind of love this place though, not gonna lie.
Bar Loose ($)
Hilarious bar/club that plays ridiculous music that almost defies classification. The times I've been there music has ranged from Native American hippie electronica (yes, imagine what that sounds like) to oldies (we're talking Ritchie Valens and oldschool love songs). The entrance fee is around 5euros, which you oddly pay straight into the bouncer's pocket as you enter the doors and coatcheck is included. The entire top floor is a large bar (usually quite loud) and the downstairs is the dance floor with additional bar. The crowd here tends to be quite young as well (early to mid 20's) but sometimes can be a little bit older and gets out of hand quite easily. Have had some hilarious nights just people-watching here. For whatever reason we've gone back here several times, despite the strange music. Drinks are very mid-range here (around 7-8euros).
Only came here once during the summer when they were opening up for a special electronica event and once was all I needed. Music, as mentioned is electronica (which was house, the time I went). Crowd is exceptionally young - we're talking late teens to early 20's. And that's it. The men who go there are a little bit older (as you could imagine, with a crowd of young girls) and people's dress code ranges from extreme club wear (what you'd expect in any normal club in the States, so heels, short dress and the works) to jeans and tshirts (the guys). Drinks were in the high-mid range (around 8-10euros). There was an entrance fee, but it was somewhere under 10euros. I went during the summer, so no need for coatcheck. Huge venue with a large dance floor and several rooms. Too bad the crowd was so difficult otherwise it could have been amazing.
Navy Jerry's ($)
Fantastic place owned by the same guy who owns two of the bars in the list below (Bhangra and Llamas). Has an old-timey American sailor feel and only plays oldies. Incredibly small dance floor and tends to attract a slightly gay crowd but have met some great people here. The entrance fee is minimal (maybe 4euros) and includes coatcheck. Drinks are high quality here and tend to be very reasonably priced (though admittedly I've not paid for my own much at this place) - somewhere around or slightly under 10euros. Great place to have casual fun. Also very conveniently located in the middle of Kamppi.
Where it all began and ended for me. A wonderful bar with good ciders on tap (usually three flavors are always available - things like blueberry, lingonberry, and pear), fantastic assortment of world beers, and even some whiskey flights. Food is also available and quite good, especially the toasties (toasted sandwiches). Dark, intimate atmosphere perfect for good conversations. Closes around 2am. Had many a good drink and night here. Avoid ordering the nachos - the nacho cheese is cold. And very disappointing. Everything else here is pretty good.
Liberty or Death ($$)
As you could probably tell by the number of times I've written about this place, I'm smitten. A modern speakeasy with a wonderful dark atmosphere and fantastic drinks. Their tailor-made drinks (changed weekly on a printed menu) are 11,50euros a pop, but they also have beer, wine, and normal mixed drinks of your choice (all for less than their specialty cocktails). The waiters and bartenders here are super nice (not to mention in period dress) and though the venue is small, it feels cozy rather than inconvenient. Would highly recommend this place.
Shanghai Cowboy ($$)
Technically a restaurant but has a great drink menu, so thought I'd include it. I had a cucumber drink last time I was here and it was divine - Finlandia vodka, cucumbers, cilantro...refreshing and great. Normal cocktail prices (10euros and up) but worth it. They just recently switched locations (used to be in a small boutique location near the harbor market, now in a larger venue where Motellet used to be) and I'm not a huge fan of their larger digs. Feels like some of the atmosphere has been lost. But the food and drinks are still amazing, so, perhaps that's good enough.
Bar Llamas ($)
Bar Llamas is one of my favorite bars in Helsinki, though unfortunately one that was not convenient to get to for me, so not one that I frequented. Owned by the same guy who owns Navy Jerry's, its theme is extraordinary on the inside - there are swings so you can sit leisurely at the bar, and everything is covered in Mexican paraphernalia - Day of the Dead, luchador masks, paintings, knicknacks of all kinds. This guy may be a little manic in his decorations (this is what we call "crafty clutter") but I love it. Felt like a little slice of San Diego home when I first moved here and couldn't find anything to relate to. They also serve amazing beers with tequila in them (single bottle, prepackaged). Drinks are reasonably priced, around 7euros each.
Bhangra Bar ($)
Another bar owned by the same guy who owns Llamas and Navy Jerry's. This one is also outrageously themed, but this time covered in Indian motif - there are fantastic Indian print fabrics everywhere and it's decked out like it's the raj's harem. There are tire swings in front of the bar this time instead of the wooden swings that are present at Llamas, but they're still just as good. This bar was just a block from my house so I was here a few times. Drinks are just as cheap (7euros or so) but mostly I was here for the atmosphere and a good place to meet friends.
Ive been to many more bars than the ones listed here, but I've listed only the best and most memorable. Should I come to think of more I'll possibly update this post or post anew about them. It's likely I've already mentioned all my good bar and club experiences in previous posts, so, if you heard me talk about it, then you know what I think about it. :)
My general comments about clubs in Helsinki: hit or miss. Go with an open mind, don't expect to have the same experience as you would in any other normal large European city, go with friends you love, and you'll do fine.
Bars in Helsinki: most bars are pretty good actually. The drinking culture is huge here so this shouldn't be shocking. Beers on tap are pretty tasty and fairly inexpensive (being the drink of choice of most men) and the ciders are also pretty nice. It may not be cheap to the rest of Europe (minus some of the other Nordic countries) but hell, you're in Finland, what can you expect. :)
Hope you can have a good time based on this guide, and thanks Helsinki!