I love freebies, and if you like your freebies, then you will like this story. Steam is Valve’s game distribution platform for the PC. Some people actually consider it the iTunes Store of gaming on the PC. Of course, the good people at Valve have created a Mac OSX version of Steam which is available for the Mac. When Steam came to the Mac, Team Fortress 2 was one of the major titles that were made available for the Mac when at the launch of Steam for Mac.
Valve have made Team Fortress 2 available as a Free-to-Play game. That means that there is no charge to download the game and play it for free. However, I suspect that there will be upgrades which you can buy whilst you are in the game. This means that the game is free-to-play, but pay to get the upgrades. Of course, this game will be free-to-play on both Windows and OSX PCs, so I suggest you download Steam, create a Steam account and download Team Fortress 2 right now.
If you are on a Windows PC, you need to be running Windows XP, Vista or Windows 7. If you are on a Mac, you will need to be running Mac OSX Leopard (Version 10.5.8), or Mac OSX Snow Leopard (version 10.6.3 or later).