I searched for a thread on this and was surprised to find none, so here it is.
All Mod info and links here: http://www.moddb.com/mods/the-haunted
Basically it is a co-op shooter like Left 4 Dead, only it takes place in large arenas rather than linear levels. It is 3rd person but has excellent shooter controls, sort of like the newer Resident Evils but on speed.
I played a couple hours tonight with a friend and we agreed it was pretty awesome, though it really needs some kind of campaign mode. The mechanics and graphics (forcing AA works just like UT3 proper) are stunning and well done, but the action gets repetitive when you have no real goal to reach. Unfortunately the mod team has posted on the Mod DB forum that they are a small team and likely can't create a campaign mode.
There are a lot of user made levels on the official UT3 boards though, so people seem to dig the game a lot. I found some levels that were really, really well done, like a lighthouse map and a bayou map.
Try it out, let me know if you like it.