A highly recommended game (make sure you play it before Portal 2).
If you want something amazing on steam, then I highly recommend this game. Besides the main game, portal has Advanced levels and challange maps. The game being so short gives you a very high replay value. This game is set in the Half-Life universe and is a puzzle game, using portals.
The game starts out with tutorial levels, basic instructure. Level 13-18 are the real challanges, after that you're given challanges outside that of the testing chambers, all leading up to a rememberable boss fight with GLaDOS
You must get this game if you do not have it already, its an amazing game.

A highly recommended game (make sure you play it before Portal 2). If you want something amazing on steam, then I highly recommend this game. Besides the main game, portal has Advanced levels and challange maps. The game being so short gives you a very high replay value. This game is set in the Half-Life universe and is a puzzle game, using portals. The game starts out with tutorial levels, basic instructure. Level 13-18 are the real challanges, after that you're given challanges outside that of the testing chambers, all leading up to a rememberable boss fight with GLaDOS You must get this game if you do not have it already, its an amazing game.