Normally when we play a game, it's either bought or pirated. Except flash-games. They are retarded.
Anyhow, for those of you who like to stay legal - or have simply played all the existing games already - here's a few games worth spending time on:

1. The NameLess Mod @
This is an awesome modification to Deus Ex (which itself is an awesome game, Steam charges 9,99€ for it) in which you play a dude named Trestkon. You are in a place called Forum City, and a moderator has gone missing.
Complete with new models, maps, textures and weapons as well as conversations (with audio) - this mod is one of the best ones I've ever played. Download!

2. AlienSwarm @ Steam
Normally, you need a game to play a mod - but not for Alienswarm! Go get it for free via Steam and kick some alien butt. With only one campaign, it's a short mod, but still worth the diskspace. More and more addons for this modification are comming in, such as survival maps(called holdout) and new campaigns.
You control one out of four marines (8 to choose from) and the objectives are decided by the map.

3. Leuge of Legends @
Important note: there is diffrent registration sites and downloadable clients for EU and US.
Leuge of Legends, or LOL for short, is a Dota-based 3rd-person battlegame. You control a unit, called Hero, and the goal is to destroy the enemy base, called Nexus. Each team have five heroes.
You have four abilities, just like Dota and HoN, as well as two summoner abilities. You choose from abilities such as heal, clarity(mana resore), flash, exhaust(blind and slow) and smite(huge damage to creeps and pets).
If you have nothing to play, give LoL a try. It's a great game.