Sierra pulled the plug on several of it's online gaming servers (sometime in November 2008 I believe), among those games was Tribes2.
However, the good people over at:
have been working hard since the authentication server went down to get a new and improved authentication server up. This one is completely free to everyone!
They finally released a patch and you can download the full game as well as the new patch and once again play one of the most epic online game ever conceived.
Also if you are new to the game welcome! and keep in mind that this game has a very steep learning curve, and please don't mind the low graphics it's a very old game. Here's a video on youtube to give you a taste of what the game is like so you can decide if you think it's worth a try.
There is still quite a community of players. Even when the authentication server went down people still found a way to play. Also, keep in mind that this new server patch is in BETA stage. Meaning they just released the basic server so that we can continue to play while they finish it up. Eventually we will see a return of the in-game browser (for viewing/editing player pages and clan pages), the in-game IRC chat rooms, and they may possibly even bring back the in-game forums.
If you enjoy Tribes2 and want to see the community grow, then please make similar threads on your favorite forums to spread the word around. Thanks.