Hello, I'm currently working on a MobArena sytem for palyers to you without any mods/plugins (redstone FTW!).
Now I'm good with redstone, but not with designing a cool arena. That's why I'm looking for someone with cool ideas, and who want's to build it for me. If interested, leave a comment and I'll mail you =)

check out the project here:
MobArena V0.1: http://www.planetminecraft.com/project/mobarena-v01-a-real-arena-fight-for-spsmp-configurable-no-mods-or-plugins-needed/
MobArena V0.2: http://www.planetminecraft.com/project/mobarena-v02-fully-configurable-spawning-behavior-your-arena-waves-no-modsplugins/