10$ game?

Terraria, Magicka

http://store.steampowered.com/sub/4340/?snr=1_7_7_230_150_9

FTL, Dungeons of Dreadmor, Awesomenauts

May I suggest Audiosurf?

Fallout 1 or 2

The Binding Of Isaac

Jet Set Radio HD.

Half Minute Hero, Freesia (been waiting for that game for many many months), Kotor I or II

Dungeon defenders

Payday. Its on sale right now.

FTL

FTL

Half Minute Hero

As suggested by the others, FTL is pretty good imo. Pretty addictive roguelike, but kinda tough (to me at least). Just note that if you want to get FTL, you can get it from their website directly for the same price, but have a DRM free version in addition to a steam key.

Aquaria.

I have never played it, don't own it, and have a burning desire to have it.

But I've heard it's good.

Terraria if you have buddies to play with, otherwise Half Minute Hero or Magicka.

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FTL, Terraria or Payday the heist+wolfpack dlc or without.
All of these in my opinion give you lots of fun for 10 bucks.