Frozen Synapse [Steam store page]
A challenging turn-based squad tactics game, though sometimes the previews are completely wrong. Sadly, game invites pop up even when you're marked unavailable or playing single-player. You must completely exit a level to adjust resolution or sound.

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

Counter-Strike: Source [Steam store page]
More realistic and slower paced than some other shooters. The guns also tend to be less accurate than your normal video game for realism, and shooting while you run is nearly impossible. Still, a reasonably fun multi-player FPS.

Age of Empires II HD [Steam store page]
One of my favorite RTS - though it does suffer from the rock/paper/scissors syndrome once you know the game well enough. Age of Empires II: Conquerors Edition was a great expansion to the original game, and it's bundled into this HD remaster.

The main benefits are that this runs nicely on high resolution - the original was locked at something like 1024x768, and this also has a few tweaks like allowing higher unit maximums - raised from 200 in the original to 500 in this remaster.

I'm normally not a huge fan of re-releasing old games, but this one was such a classic, and the small tweaks they made improve the experience without ruining the nostalgia.