BattleBlock Theater is a great co-op platformer that's pretty silly but still fun and can be challenging. You can play it solo, too. It's on sale for $3.
If you haven't played it already, I highly recommend Borderlands 2 GOTY, which is only $9 (not a historic low, but close), and I have about 200 hours in it. Great if you enjoy shooting things and random loot systems.
Braid is a great indie puzzle game that's only $1.50 right now. Time manipulation-based platforming and stuff.
Dust: An Elysian Tail has a great combat system that I found very, very enjoyable. Though the plot isn't too great and the voice acting is lacking, it is an indie game, so those things are understandable, and it's still pretty fully voiced, which can be nice. Some people have an issue with the art style and stuff, but even if you think it's weird I think that's a silly reason to not enjoy this game. Only $3 right now.
Crypt of the Necrodancer is just under $4 and is great if you like rhythm games or roguelikes, since it's both!
Mark of the Ninja is one of my favorite indie games, a stealth action platformer kind of thing, and is less than $4. An absolute steal.
Dishonored is a triple-A title, but it's only just over $3 right now, and is one of my favorite games as well - great stealth gameplay and lots of neat abilities and playstyles that you can incorporate.
If you like RPGs at all, or like Star Wars at all, and haven't played the KOTOR games, get them. They're only $2.50 each and they're fantastic.
As far as roguelikes, FTL and Spelunky, while fairly different, are both quite good and only $3 each. I got more into FTL personally, since the platforming in Spelunky didn't engage me as much as the space combat and strategy in FTL.
Hotline Miami is a great frantic top-down shooter with a story that ended up being a good bit more than I expected. It has a great soundtrack, too, and is only $2.50 for an indie classic.
One Finger Death Punch is one I don't hear a lot about, and is kind of a diamond in the rough when it comes to that developer. It's cheap, with nothing but stick figures, but man is it satisfying to play. Twitch reflexes and just a touch of strategy and almost rhythm game-esque gameplay make it feel smooth and engaging even though there's only two buttons used. It's only $1.24, and is an excellent diversion. Especially in class.
So that's a decent dump of games I can recommend, mostly on the cheaper/indie side of things. I can go on for quite a bit, but if you're looking for anything in particular I can probably recommend something more specific, too (I have an average playtime of almost 15 hours on about 250 different Steam games so there's a lot I probably have some sort of opinion on).
