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.

Costume Quest 2 [Steam store page]
Better environments than CQ1, with the same great characters. The first level is the only bad part, so suffering through that small grind is worth it.

This slick looking puzzle game is simple, but enjoyable. As the game progresses you unlock six special abilities and more complex puzzles to go with them. Aside from the simple 'normal' puzzles, there are also 'hard' and 'bonus' levels.

Crayon Physics Deluxe [Steam store page]
The concept is interesting but the puzzles are super simple - though you can get extra stars (points) by finishing them in overly-complex ways. Would have been cooler if drawing shapes spawned objects like in Trine (with more variety)

This nicely polished sliding puzzle game is fun yet challenging. Solving the levels in the correct amount of moves is simple, but the time limits are amazingly difficult if you choose to attempt them. A huge amount of different levels to complete

Plants vs. Zombies GOTY Edition [Steam store page]
A fairly relaxing arcade game - while repetitive it can get tough on the endless levels, especially right before each of the endless achievements. Would be nice if the game remembered your choice of plants between levels

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