Steam is a game and download/update manager from Valve, the creators of Half-Life.
Steam gives you full access to the way you’ve always wanted to play games.

Buy and download full retail games, from new releases to classics. Check out demos and trailers of upcoming games. And make yourself at home in a community built by gamers, for gamers.

It’s free to join and easy to use.

Features:
* Instantly access your favorite games
The full Steam library and all future Steam releases are at your fingertips. After you've purchased a game, download it immediately and start playing.
* Join the Steam Community
Now it's easy to find someone to play with, meet up with friends, connect with groups with similar interests, and host and join chats, matches, and tournaments. Best of all, it's all free.
* Chat with your friends while gaming
With Steam's "Friends" service, you can talk to your friends while you play. You can also see when they're playing games and easily join the same servers together.
* Easily browse multiplayer game servers
Steam's integrated server browser makes it easy to navigate thousands of Internet game servers.
* Receive automatic game updates
Hunting for patches and downloading from unorganized web sites is so twentieth-century. On Steam, your games stay up-to-date by themselves. No hassles.
* Play your games on any PC
Once you have a Steam account, you can sign in from any PC to access your games. Your games are associated with your account, not your computer.
* Enjoy Steam-only special offers
From pre-release discounts to free-play weekends, Steam delivers what gamers value most.