This is a new mob-farm concept I came up with (at least I have not seen it before). It uses rails & a minecart system and darkrooms/spawn pads that are flushed with water. It "switches" on and off automatically. It does not take too many recources to build this, so it is reasonably easy to build it in survival mode.
You get about 7680 drops per hour from it. That includes around 27 stacks of gunpowder! It works in Minecraft 1.4 and pretty much all the recent updates.
It is fully automatic, and with the addition of hoppers in the snapshots and 1.5, it should be possible to automate it even further.
If you have any improvement suggestions, just post them in the comments or leave a video response!