Only just started actively using Steam and, up until a couple days ago, only had Portal, Team Fortress 2 and Eets (because they were free). This sale has been excellent - I have purchase quite a few games and plan to purchase at least a few more.
Bully: Scholarship Edition Had always wanted to play this, but I could never find it for less than $20. When it went on sale for $3.74 I had to have it. Unfortunately, it is Windows-only and my only Windows machine is unable to run it. I am going to try to set up a Boot Camp, but I don't have anything except XP, not sure if I won't just wait until I can afford to buy a copy of Windows 7. Still, though, it was a good deal and I'm not all that unhappy.
Left 4 Dead 2 I only recent bought the first game for the XBOX 360 and really enjoyed it. I had been debating purchasing this all week at the $14 price, but I waited it out and it went on for $5 today. Very impressed with this buy.
Garry's Mod Have always wanted to play around with this and only a couple weeks ago I had it in my cart, ready to purchase, at $10. Good thing I waited, $2.40 is excellent.
Machinarium Loved the online demo, and I know people who have paid insane prices for it. None of them believed it was $3.40 when I mentioned it to them after buying it today.
VVVVVV Probably the most inventive and well-scored game I have played in a long time. Incredibly appealing to gamers who want nostalgia, a fun play-mechanic (no jumping, just gravity switching) and a great soundtrack. Definitely worth the $2.40, if not more.
Sonic 3 & Knuckles The only other Windows-only game I got. I never was into Sonic as a kid, but I keep hearing that this was the best Sonic game for Genesis. Figured I'd give it a try for only $3.34. Also debating on getting Sonic Adventure DX - $6.02 right now, definitely will get it if it goes down.
On the Rain-Slick Precipice of Darkness Both episodes for $1.80? Can't complain.