Steam is a game and download/update manager from Valve, the creator of Half-Life.
Allowing you to buy and download any of Valve's games, like the recent, fantastic Left 4 Dead 2, Steam also has a huge library of titles from other big developers, as well as free demos and videos. The list of games is ever increasing, and it also supports non-commercial things like game mods.
Installing and setting up a Steam account is really easy, and the download manager is very stable. Whatever your internet connection speed, you don't need to worry about stopping a download partway through. If you exit Steam, it will continue your downloads the next time you start it without complaints.
While you miss out on a box or instruction booklet by buying games from Steam, you do get all the latest patches and updates downloaded automatically. There are often exclusive demos and offers available through Steam too, and there's a great community service which allows you to join groups and find people to play online. Its major downfall is that the client auto-updates far too often.
This new version of Steam has a much improved interface, with a better news stream and cleaner overall feel. The layout is much simpler, allowing you to quickly navigate between games, available demos and videos. There's a game wishlist feature, and your game library now has real time search and customizable categories.
Steam may not sell every game under the sun, but it's an excellent download application featuring some of the best PC gaming you can find, and the best community features found in PC gaming.
Added Big Picture mode which is optimized for using Steam fullscreen with a Gamepad on HDTVs
Games managed by the new content system can select alternate installation locations other than the Steam folder (Windows)
Fixed the in-game overlay not recognizing correctly which game is running sometimes when running a non-Steam shortcut
Fixed some potential GDI handle leaks in the Steam client (Windows)
Fixed Steam Cloud occasionally not syncing files it should
Fixed a bug where entering a password-protected content beta without a password would act like it succeeded even though it hand’t
Improved TGA validation in image loading code
Improved steam:// URL validation in some cases
Fixed a new in-game overlay crash in very specific scenarios on Windows 8 with legacy DX9 games
Fixed problems uppercasing some characters (e.g. Greek terminal sigma)
Fixed error retrieving third party cd keys when launching games on OS X (OSX)
Fix the in-game overlay to correctly inject when a launcher calls ShellExecute with an empty string for the verb (Windows, fixes SWTOR)
Improvements to the change email wizard