One of those ultra-rare Linux gamers? Then check out this page where you can sign up to beta test the Linux edition of X3: Reunion, the latest entry in the space combat series. On LinuxGames are details on participation.
For Windows gamers, there’s the Hellgate London beta. However, this won’t be free, as interested parties will have to pre-order the game through Hellgate Resistance, who notes that buying the game will include a chance to participate in the game’s beta testing: “Signing up for the pre-order will also automatically register your name on the waiting list for the Hellgate: London beta test.”