Valve just released the new Gold Rush update and to promote it they're having a free TF2 weekend from May 2-4. You can preload TF2 on Steam any time this week and it will be unlocked at midnight on the 2nd.
Link.
Edit: It's going to be unlocked at noon PDT on Friday, not midnight.
Edit: Title update to show it's on.
Link.
Edit: It's going to be unlocked at noon PDT on Friday, not midnight.
Edit: Title update to show it's on.