As one game leaves open beta (opens in new tab), so another can enter. I think that's how it works, anyway. The game in question is Dirty Bomb. It's a team-based free-to-play FPS from Splash Damage, the makers of Enemy Territory: Quake Wars and, yes, Brink.
For more, check out the trailer below.
Of Dirty Bomb's initial roster of 12 mercenaries, five are currently available to play for free. Two of them—medic Aura and the air-strike dropping Skyhammer—are permanently unlocked for all players. The others are drawn from the mercenary pool, and made free for a limited time. The current free rotation consists of the orbital-laser-striking Kira, frontline support medic Sawbonez, and the gun-'n-grenades loving Fragger.
You can play Dirty Bomb right now, through Steam (opens in new tab).