For those of you who have ever wanted a wave-style or endless mobs battle room on your SMP server:
Firstly, you'll need several mob spawners for each type of mob you want to battle. Use water streams to merge the mobs into one stream for each type of mob, and then use that stream to send them to your combat room.
The combat room must be an enclosed, empty room that is at least 8x8x8 large. In the sides of the walls are examples of what I like to call: Elevated Mob Transport System.
The Combat Room should have an impenetrable floor of adminium, and a glass roof. You can choose to light the room or not; it's your choice.
Above the Combat Room is the Control Room. This room controls when and what mobs are attacking the players below by using an array of levers that act as door controls. The doors decide whether mobs are allowed to enter the Combat Room. A diagram of the mob doors is below in the Control section.
Additionally, the moderators may wish to have something like a Modern Warfare 2 Emergency Airdrop: A public storage cart filled with food, armor, and weapons that can be dropped through a hole in the glass floor to provide the combatants supplies.
Elevated Mob Transport System - This is a series of diagrams outlining the important areas of this Mob Transport System.
= Any block unaffected by gravity (and not glass)
 = Air
= Water Source
= Water Flow
= Mob Spawner
Control - Where the mob streams converge and where you control mobs going into the combat room.
(Top down view - make sure spider streams are 2x2x1, not 1x1x2. Also, add in the wiring for the doors and levers yourself.)
Output - Where the Combat Room is.
        
Combat Room - This is the recommended layout for a Combat Room.
The rest of my amusement park. In this pic, you can see my incomplete rolercoaster, The Rage.
A view of the control room from the combat room.
The combat room from the entrance. Only one side of the mob feeder is constructed so far.
The whole structure from the outside. To the left is the combat room, while at the right is the spawners. You can still see scaffolding made of snow on the structure.
Feel free to make any suggestions to improve this system. Thanks for reading this post.
Disclaimer: All mechanisms in this post are UNTESTED. If you have tested this, please post on this thread or PM me.
This post will be updated as I find the time to test this, and add screenshots of a Combat session. Additionally, it is impossible to have something like this without having a custom server, or in SSP, hacking to alter Mob Spawners, place spawners, and get so much building material. This is a good project to undertake on an extra world, one where you build huge structures without exploring.
I will post the save later, as soon as the structure is completed. This will most likiely be around a week or two from now.