From the Netrek Homepage:
“Netrek is a graphical real-time space battle simulation with a Star Trek theme. Netrek costs no money to play, and is open-source. There is a galaxy with four races in it: the Federation, the Klingons, the Romulans, and the Orions. The game is divided into two teams of 8 (or less). Each players gets a starship to fly. Players dogfight each other and attempt to conquer each other’s planets. There are several different types of ships, from fast, fragile scouts up to big, slow battleships and starbases; this allows a great deal of variance in play styles.
The purpose of the game is to conquer the galaxy by taking over all of the enemy race’s planets. To do this, your team must bomb most of the enemy armies off of the planet and beam down enough of your own armies to finish off the remaining enemies. Ships that have killed enemies can beam up their team’s armies to carry to the enemy planets.
Normally, you play against other humans, but there are practice servers with a few robot players on them (bots) for training or just when no-one else is around.
Netrek was the first Internet team game, ever. It can in many ways be compared to both Chess and Baseball. This isn’t a game you master in a week and throw on a shelf; getting good takes time, and there will always be new challenges. It doesn’t look very flashy; Chess doesn’t either. It’s about the game. It’s been honed over a long period of time to be challenging and exciting, to balance fast reflexes and fast thinking, to be complicated enough to be interesting, but simple enough to play, and it’s elegant, even beautiful, to watch when played well. Once you learn the basics, you can be as serious as you want to be about it; drop in once in a while for a pickup game or put in so much time that people will wonder whether or not you’re a human.”
Link to a web “metaserver” – basically shows the status of some running servers, though I think you can find more games from inside the running client.