Valve today has relaunched Team Fortress 2 as a complete free game, in what Valve describes as "the most ambitious update in the history of Team Fortress 2”.

full interview: http://www.develop-online.net/features/1325/Free-at-last

If you already own steam, all you have to do is install from this page: http://store.steampowered.com/app/440/

edit: Some more info on the free/premium account difference here.