After what seems like an eternity, the mob arena project has officially open for business again! It's located at /warp mobarena, and is not too far from the warp.
A centre area serves as a mini-base, with provisions for both repairing the arena and recovering HP, as well as discarding and crafting loot. Ender chests may be placed in the future among other things.
A light control system regulates mob spawning, and is on by default. Turn it off for a little bit for the mobs to spawn, then click it back on and slay what came in!
Plenty of slimes come in here too, and they'll spawn regardless of lighting so it's also great for getting those slimeballs. Often they may spawn up in the light system level - disregard them while a fix is being considered.