There are a handful of absolutely amazing ones out there now. I'm feeling too lazy to find links that I didn't memorize right now, but here are a few worthy of mention:
Alien Swarm:
http://www.blackcatgames.com/swarmTo me this feels like a mix between Syndicate and Aliens. Check out the video, it gives you a small taste.
Deathball:
http://www.deathball.netIf you have played ProBall for Quake 3, you're in for a treat! Take Bombing Run, remove all weapons except for an impact hammer-style weapon, and put it in a field arena with a keeper, and it's deathball!
Red Orchestra: too lazy to find link but check it out
One of the best WWII-style mods I've seen out there. I'm extremely impressed with the weapon variety and level design. Check out the pre-release trailer on their website.
Unwheel: no link
WTF?! Unreal Racing? How cool! Put this next to Unreal Golf and you've got yourself a good time
This is just a few -- looking forward to Neotokyo, too!