Hey there,
This is my first major project type thing and it is a Mob Grinder for Minecraft 1.0.0 which has a switch in the control labeled XP Grinder that when flicked turns the Mob Grinder into and XP Grinder. The entire thing is operated from one control which includes the collection point, option switches and XP Grinding position. This is my first project and I created it all from scratch excluding the Spawner where I modified a design from the Minecraft Wiki to make it suitable for an XP Grinder and to not spawn spiders. There is an MC Edit Schematic File available to download as well as a World File so you have many options.

Comments and Diamonds are greatly appreciated.

I found that on a flat grass world with no caves this gave me 1518 items in a hour. (Calculated by setting to hard mode and dawn and then ensuring that it didn't rain or turn to night for 10 minutes. I then multiplied the number of items I got in 10 minutes - 253 - by 6 to reach 1518 items per hour)

Why not check out my channel for other cool Redstone Stuff - http://www.youtube.com/user/TaintedRedstone?feature=mhee

There will be a series of videos soon detailing the steps to make something like this right through from the spawner to the elevator to the killing device to the redstone to control the mode.

Hope people enjoy using this or find it helpful.