SW:TA is one of the longest running mod and creates a Total Conversion of TA including Imperial walkers, Starfighter dogfights, and Turbolaser Defense Batteries.
Galactic conquest is a mod for Battlefield 1942 that wll include gameplay similar to Battlefront (but something to keep in mind is that these guys came first!). A multiplayer mod that will allow players to play on either side of the Galactic Civil War or the Clone Wars and will include many of the famous Star Wars vehicles.
Troopers: Dawn of Destiny is a Total Conversion for Unreal Tournament 2003/4 that sports an exciting combination of all-new playable characters, breathtaking battlefields, completely authentic and technically realistic weaponry; and both old and new modes of team-oriented gameplay.
New Rebllion is a graphically beautful mod for Homeworld that gives a unique Alternate Universe Single Player Story line that starts with the Battle of Endor going wrong...
Wildstar02301 wrote up a very comprehensive list of mods for X-Wing Alliance, and while we aren't able to profile them all, we'd like to at the very least let you see some (if not all) of what's out there for you to enjoy.