In a new post on the TF2 Blog Valve has made a call for anyone and everyone who owns a copy of TF2 to test new builds. Some of the gameplay features and stats tweaked for the beta:
Upon entry into the beta you will get all items currently unlockable through achievements, but random drops and the store will not be functional. All items can be taken away at any time while participating in the beta. To get into the beta simply launch Steam, open the Library and install "Team Fortress 2 Beta" from your games list.
The game launched as a beta before its release way back in 2007!