You can get Team Fortress 2 by downloading Steam (A simple online game store/manager) and using that program to download Team Fortress 2. Its available for both Mac and PC.
Fair warning for those planning on downloading the game, the servers will be so backed up with other users trying to download the game. You may want to leave your computer on overnight while it downloads because it can take SEVERAL hours for the download to finish!
I also noticed on several TF2 forums that people who have paid for this game prior to this weekend. I'd be angry too if I bought the game and it became free-to-play all of a sudden.
Thats all for now guys, I gotta get back to gaming!