Hexcells Plus [Steam store page]
Basically the same as Hexcells, just adding another set of levels - since this game doesn't randomly generate them (unless you buy Hexcells Infinite).

Hexcells Infinite [Steam store page]
While the random-map generation isn't quite as challenging as it should be, I still really enjoy the Hexcells series - they're a modernized Minesweeper.

Minesweeper on steroids - this is a great take on the genre, adding a lot more elements while still keeping a good deductive feel.

This is a well polished and reasonably innovative machine building puzzle game. The interface is complex yet easy to use, and the game can be challenging. The graphics are clean and simple, but fitting. A well done indie.

Defense Grid: The Awakening [Steam store page]
A carefully balanced tower defense game with excellent voice acting that weaves an interesting story throughout the campaign.

It looks nice and is enjoyable, but could use some tweaks especially at the more advanced levels. A bunch of enemies in a row can tag team your resources quickly down the line from quite a distance away - which feels a bit cheap at times, especially since the resources travel much faster when rubber-banding to an enemy unit.

It also would be nice to have more control over tower targeting priority, or even just a manual targeting mode. For example, in the later levels you want guns to tear the shields off targets before the lasers attack them - but the guns stay focused on a target until it is completely dead even if they aren't doing effective damage.

It's still one of the better TD games out there though, excellent polish and balance.

Anomaly: Warzone Earth [Steam store page]
A tower defense role-reversal where you control the attacking units. It lacks the ability to stop or reverse your units, leaving you to determine the route and do repairs and special abilities. A bit unforgiving, requiring no casualties on normal.

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)

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

Machinarium [Steam store page]
Nicely drawn point and click - but the puzzles are too obtuse for their own good. The hint button is hidden behind a long minigame with a key that shoots bullets (which the game doesn't mention and the minigame can't be completed without them)

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