Tribes: Ascend is in open beta. You can sign up, download it and play it right now. It's already good fun, but it's still a work in progress. There are plenty more maps and perks to be added before Hi-Rez consider the game complete. The latest patch has made a few of those additions already, adding a new team deathmatch map called "Outskirts" and two new perks.
The first is Rage. Getting angry will boost your energy regeneration and offer you a 25% heal if you're standing next to your flag when an enemy snatches it, a good perk for defenders who have just been blown off the flag by a Pathfinder's spinfusor disk. The second one is a little more exotic. The Sonic Punch will send out a shockwave when you melee. This can push back enemy players and, importantly, dislodge a flag from an opponent's grasp. It'll be interesting to see how top Tribes players make use of this one.