So, it's pretty ghetto, but I'm holding a "mob arena" of sorts on SMP5. Go into the wild and follow the bedrock bridge to the "arena". Once started, I will spawn random mobs until only one person remains, which they can keep all the drops. Come and join the fun! Go into the wild of SMP5 and head South (not outpost, just walk south). You can't miss it.
Ugh. Was killed by the mean people breaking my bunker. And they lead the creepers in there. And the rest is history.
Since I am guessing your gonna want feedback, here it goes. Bother to make a real arena, first off. That freaking bedrock square is way to small, and is mostly determined by a large amount of mobs picking one target. Secoundly, you should make better viewing areas. 12 people in a 4 block wide hole isnt gonna work. XD And finnaly, disable building. It will allow for you to make a real map, as well as keep from people from randomly placing blocks everywhere, like I did.
