What is it?
MobArena is a plugin witch pitches up to four players against increasingly difficult PVE action

Why this plugin?

FEATURES
Fight oncoming waves of monsters!
Play on your own, or team up with friends!
Earn glorious prizes!
Build your own arena!
Completely customizable classes, waves and rewards!
Easy to set up!
Extremely easy to use!
Very few user commands!
Supports Permissions, Spout, and all major economies!

Where can i read more?
http://dev.bukkit.org/server-mods/mobarena/

We had This plugin but it was incredibly Buggy and easily exploited by players.

Other than those issues, it was the funnest thing ever. Period.