Well, if you have an Xbox controller, I'd suggest Dark Souls. My flatmate has it (and a controller) and seems like plenty of fun. Also, it's rather high in your wishlist, so assuming it's ordered in any way of preference, you'll probably enjoy it a lot. The keyboard+mouse controls seem like a big pile of shit, though, that's why I recommend the controller. Just as an aside - something I noticed - both Borderlands 2 and Dark souls are the same price in the UK - £29.99.
You can grab DS along with Deus Ex: GOTY and I think you should have a blast (as well as fit right into the price range)
Magicka [http://store.steampowered.com/app/42910/] - is, hands down, my favourite release on Steam. Not counting Bloodlines, of course. It's immensely fun and has pretty much the best magic system out there. Something I had been waiting for (the magic system) for a long time, the game just packs along with everything wanted to go with it. And I didn't even know I wanted precisely that.
Amnesia: the Dark Descent [http://store.steampowered.com/app/57300/] is really close behind Magicka in terms of me liking it and being on Steam. The atmosphere, the setting, the gameplay - they combine to make it one of my favourite games. I was hooked up straight up when I first saw the trailer. I remember it clearly - it was on sale and I clicked on it to see what the game was about, as I didn't know anything about it. by the time the guy hid in the closet (a bit after 2 minutes in), I was convinced to buy it, I just had to wait for the trailer to finish.
Spec Ops: The Line [http://store.steampowered.com/app/50300/] is something amazing, too. It's utterly stunning, and I think I should leave it at that.
Most recently, I've been wasting[/I] spending wisely tons of hours with Torchlight 2 [http://store.steampowered.com/app/200710/]. And I find it quite good. It's a game I can just relax and play as long as I like. Last week, I even studied while playing it - I just listened to a course related podcast as I did. Worked quite well - I managed to get through the whole hour of it in one sitting. If you even played and liked Diablo 2, T2 would be right up your alley.
Cthulhu Saves the World + Breath of Death VII [http://store.steampowered.com/sub/8848] deserves a mention. At $3 (£2) for two games, they are quite good. I really enjoyed them. CStW is by far the better one, but it costs the same as the double pack, so just get both.
The Heroes Pack [http://store.steampowered.com/sub/1753/] is another "more bang for your buck" pack. It has all of Heroes 5, along with Dark Messiah. It's one of the few instances I'd agree to giving money for Dark Messiah. Don't get me wrong, it's not a bad game (for the record, it was made by the same people who did Dishonored) just not that good price-wise. Especially since it costs almost the same as Heroes 5. Although, others may feel the price is justified. I still recommend playing some of it, since it's worth at least seeing it. Whatever the case, the Heroes Pack is quite awesome. H5 is almost my favourite one in the series. Definitely so with the expansions.
Mirror's Edge [http://store.steampowered.com/app/17410/] is a game I (somewhat) recently discovered and fell in love with. It looks absolutely beautiful and the sountrack I listened to for a week non-stop after I played it, for it's the audio version of cookies and honey. But at $20 it's slightly too expensive.
Frozen Synapse [http://store.steampowered.com/sub/14855/] - a turn-based tactical 2D shooter. Or something along those lines. But let me synthesize that description into one word - "awesome". Basically, you control a SWAT(??? or equivalent, at least) team set against another similar one. The guys are equipped with pistols, machine guns, rocket launchers, etc. It's quite tactical in nature - movement, fighting, cover, facing - they can win or lose you a combat. Also, the soundtrack reminds me of Mirror's Edge in some ways. All of them good.
Costume Quest [http://store.steampowered.com/app/115100/] is quite thematic right now. It's a Halloween themed RPG aimed at the younger audience by Double Fine. I still enjoyed it very much. And it's on sale right now! I suspect it would be on sale again on Halloween.
Albeit last in the list Vampire: the Masquerade - Bloodlines [http://store.steampowered.com/app/2600/] is my favourite game on Steam and almost ever. I'll still say $20 is too much for it though, to avoid negativity, I'll just say that this price may put people off from getting it even if they should. On the other hand, Halloween is coming and Bloodlines would almost certainly be on sale then, along with other horror-y games.
