Team Fortress 2 [Steam store page]
I wanted to love this game, but found it hard to get into because it requires team effort (mainly a healer at your back) or you die exceptionally fast. Unlike Unreal Tournament or Counter-Strike: Source where you can easily play a quick lone wolf match

Minecraft in 2D, but with good combat and bosses. You can make a large variety of weapons and potions, and there are sky islands, dungeons, and the underworld to explore. The 1.1 update adds tons of content and improves the game considerably.