Team Fortress 2 Beta
The TF2 Open Beta is a beta version of the game open to all TF2 owners, expected to be announced in the near future. The Open Beta would be used to test various changes to the game and observe their effects on gameplay, before integrating certain changes into the game.
Robin Walker has mentioned that the main goal of the beta would be to test how systems in the game affect each other and are affected, and that experiments would be "wide scale".