I played Portal to the end and I love the game, it's awesome. You can buy it online for $17.95 without having to purchase anything else ( http://www.steampowered.com/v/index.php?area=game&AppId=400&cc=US ) and buying it from that domain means that the money is going straight to the game developers, not through a 3rd party publisher, etc.

Don't think it works for a mac tho. :|