Train Valley [Steam store page]
Starts out simple, but to be really good at this game you have to be able to multi-task incredibly accurately and quickly. That just isn't me, but the game is well designed and will be fun for people who love the challenge of optimizing their strategy and reaction times to climb the leaderboard.

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.

World of Goo [Steam store page]
I'm not madly in love with this game, but it's a decent way to pass the time. The puzzles are usually fairly easy, though sometimes it is hard to execute them because of the way the AI makes goo run around the structure