As for games, let's see.
There's Valve's games, of course. My first purchase for Steam was the $99.99 Valve Complete pack (2007 collection), since I wanted Portal, and it was not available alone at the time. I could have gotten the Orange Box, but I got so many more games for a better value this way. For you though, unless you want more of their older games, I'd just grab the Orange Box. You get Half Life 2 (and both Episodes), Portal, and Team Fortress 2.
Unless you're into Counter Strike (I imagine you'd have it by now if you were) or Left 4 Dead series, that'll get you most of their better stuff for cheap.
Amnesia, the Dark Descent is said to be good/scary. I've actually been too afraid to fully start it yet. Don't look it up on Youtube (like I did a bit) and ruin it. You're a guy in Europe? U.K. somewhere who lost his memory, and you go through the game to learn why. You don't have weapons. You just run. Try the demo.
Audiosurf is, like the name suggests, a game where you "ride your music". It assembles a track based on your song, and you collect boxes to fill in a bracket and each character has different abilities to aid. It's all for getting a high score (there's a leader board), but it's more a game just for fun. Old, but original and unique. It's also cheap. Again, has a demo.
If you haven't tried it yet, Batman Arkham Asylum. Has a demo, and was one of 2009's best.
Blade Kitten is an anime style 3-D sidescroller platformer. I haven't fully played it, but tried the demo and bought it. It has a demo to try.
Command and Conquer 3 Tiberium Wars is my favorite recent Command & Conquer. Unless you're into it though, it's not a must have. There is a demo, but not on Steam.
I believe you have the Crysis titles, so I skip those. I don't recommend Crysis 2. I'm skipping it.
Dead Space is good. Dead Space 2 (which I don't yet have) is said to be more of the same, but that's a good thing!
Doom 3 and it's expansion pack, Resurrection of Evil, are both worth their playthrough. Old, but definitely a staple, and still good looking (but dark). It's a decent story, and while it's a corridor shooter, it's a very, very good one. Steam has the demo, but the demo for Doom 3 doesn't run on Windows Vista/7, so sadly, you can't try beforehand. The full game works on the newer operating systems though. They just never patched the demo. If you have a Windows XP setup, it'll work there.
Far Cry (the first) is cheap ($10), and like the above, an older, but staple of the times, title. It's Crysis's spiritual predecessor. A bit more "cheesy" in some areas, but still a classic. It has a demo, but I don't think on Steam has it (I could be wrong).
Far Cry 2 I have and it's "okay" but NOT either a true sequel or a great game. Crysis was the spiritual successor since CryTek made Far Cry and Crysis, but Ubisoft owned the Far Cry IP so they made Far Cry 2.
There's the Grand Theft Auto games for some classics, but I hardly touch mine. Good to have, but only must haves if you know you want them.
Just Cause 2. A HUGE and beautiful and FUN open world. One of my higher recommendations. It has a demo.
Mafia II is worth a mention. Check my profile for my short review/recommendation. It's a slight lacking in replayability, and it's quite mature in some areas, and it's got a slight console feel, but hell if the story and setting didn't absolutely drag me in, and check my profile to see my hours put in (that's one and a half playthroughs worth), so it's worth it. It has a demo to try it.
Mass Effect 2 seems good (Game of the Year potential), but I only tried a bit of it. It seems it suffered a bit compared to the first (less depth and RPG elements for streamlining), but it's still highly recommended. It has a demo to see if you'd like it.
Osmos is a niche and indie game that's cheap and short fun. It has a demo to try.
Peggle and Peggle Nights are easy hour eaters and cheap. "casual games", but hell, I highly recommend them myself and that is saying alot. I think both have demos (at least the original does, but the Nights version is just more of the same).
Plants vs Zombies. Now. That is all (it has a demo). Really, perhaps one of Popcap's best, and it has an awesome trailer.
I already mentioned Portal in the Orange Box. Portal 2 is coming up too. It has a co-op campaign too (pre-order nets you a free Portal 1 copy).
Here's another indie title. Shank is a 2D sidescroller beat 'em up. It's that simple (but in these days, that makes it fall short in some reviews). Cheap, and VERY challenging. Nice artistic style and has some depth, but it's still simple. It's said to be short, but it's hard (even on normal difficulty, no easy), so it adds some hours. I still haven't beaten it. Recommended, but only if you have a game pad. It has a demo.
If you like Diablo type games, Torchlight will eat your hours. I never fell for Diablo, but still racked up hours in it. It has a demo to try. Torchlight 2 will come at some point too.
Trine is an awesome 3-D sidescrolling adventure with a beautiful world and 3 character switch gameplay. It, again, has a demo (most of them I'm mentioning do, and I own them, must be why! games these days stop doing demos so I can't try them, so I don't necessarily buy them...).
VVVVVV is a short (I got nearly 8 hours out of it, that's counting going back though for the extras), but cheap/fun top tier indie title.
That's just a "short" list. Don't forget to check my profile for everything I own/play, and check all of the free games and MODs Steam has (many for the Source engine/games).