Orcs Must Die! [Steam store page]
While not co-op like the sequel, this game has an excellent sense of humor and decently balanced maps. Too bad the humor didn't carry over to Orcs Must Die! 2

Orcs Must Die! 2 [Steam store page]
They added co-op, but lost some of the humor that made Orcs Must Die! so fun. Similar gameplay, with more expansive levels to challenge two players.

HunieCam Studio [Steam store page]
It seems like a lot of people like this game, but it's average at best, with the quality declining the more time you spend playing it. Basically it's a shallow clicker game, which surprised me since the store page seems to imply a bit more strategy and management.

Instead it just tries to keep you occupied clicking a handful of different buildings instead of adding any real depth. I'm sure you can optimize it slightly to get a higher score, but there aren't even semi-deep strategy or mechanics here.

As a side note, this game is tagged 'Nudity', but does not contain any as far as I can tell - unlike other games like HuniePop from the same creator.

Saints Row IV [Steam store page]
While Saints Row: The Third was fun madness, this sequel adds a superhero twist with great new abilities and much easier travel than the previous game.

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.

The casting system is reasonably intuitive, and the gameplay itself is as fun as Torchlight, though the heavy corridor feel of the levels detracts a bit. There is a smattering of story and humor throughout, but I wouldn't call it story-driven.

Killing Floor [Steam store page]
The graphics are a bit rough but this game is all about speed and accuracy killing waves of experimental creatures. Forget story, the co-op is loads of fun with a group of friends, and fairly challenging as well.

The most difficult part is the way enemies rubberband - if you run full tilt on many maps, and turn around they'll still be very close behind you - rounding the nearest corner or doorway. The spawn locations also don't seem to be set specifically - they seem to appear directly around any corner that you haven't looked around in the last few seconds, or further ahead if you're running rather than camping. On some areas in maps they also drop down from above, off buildings or through holes in the roof.

These mechanics aren't necessarily flaws - they make the game much more fast paced and difficult than it would be if you could kite them more strategically. Kiting is still very much possible because the game limits the number of enemies on the map at any time - so if you can group them all behind you then they will slowly appear in front of you as you kill ones behind.

Expansive levels that make Portal look like a tech demo, with quite a few new fun mechanics and a lot of back-story about the Aperture Corporation. It's about three times as long as the original, and once again ends with fantastic music.

This was fantastic the first time through - physics puzzles with tiny details that draw you into the story. The only reason I can't give it 10/10 is that Portal 2 is even better.

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

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

Just Cause 2 [Steam store page]
Something for everyone - a sandbox with over the top violence and stunts, plenty of collectables, challenges, and a story line that appears just often enough to keep you interested in the missions. Steal helicopters and parachute from planes...

Borderlands [Steam store page]
This is a weird mix of MMO leveling and FPS, and in a certain aspect it works okay, but at other times it can be frustrating.

The beginning of this game includes a lot of scripted sequences, which can be a bit annoying if you play through multiple times, as was intended (you reach level 35 or so on the first playthrough, 50+ on the second). The first time playing it seems incredibly smooth and well paced, teaching you all your necessary skills painlessly.

The game itself isn't hard - I was level 13 before I died the first time - but after a while it gets annoying to take headshots and not actually kill the enemy, like a normal FPS. This is due to the level mechanic - killing an enemy one level above you is difficult, and two is nearly impossible. The exception is playing the tank/melee character, who has a special ability that flattens higher levels with ease.

There is also a melee enemy class known as Psycho that has an annoying tendency, when injured, to run full tilt at you holding a hand grenade above their heads. Many times when this happens they are inescapably close, which results in your instant death.

Lastly, the bosses tend to take 3-10 minutes for a solo player to slowly tear through their massive health pools - almost like fighting an MMO raid boss on your own. Almost every boss also has a group of annoying normal enemies to dispatch during the beginning of the fight as well. These encounters are probably much more suited to the co-op gameplay.

Overall a mixed bag, fun in certain aspects, but also one of the few games that makes me feel angry when I play it. That's a sure sign the enemies aren't quite as balanced as they should be.

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