World Domination by Fingolfin, still in development but nonthless it's playable in both singleplayer and multiplayer, and offers some fun tech to play with.
TcX, it's pretty much Quake but you use a bird's view cam instead of first person.
Naval Battle & Battletanks, the only AoSes I find fun and interesting.
RDZ Industries, a pretty challenging total conversion.
Metastasis, one of the more fun Space Station 13/Mafia-esque maps.
Fantasy Life, a pretty well-made LOAP-style RPG.
Real Life [For Morons], another great LOAP, less fantasy, more humor.
Founders of the North: one of best builder maps out there.