Guide for Smart Shifting Floors:
http://www.youtube.com/watch?v=-wYyal7G6LIThis is used for collecting spawns. You put this over an area mobs will spawn, maybe a specific kind. When the mob spawns, the floor will shift, dropping the mob into whatever system you have. This works by tripwires to detect the mob, then shifting any effected sections of the floor until the mob has left the spawn system. It will move adjacent floor tiles to assure a mob does not trap in the system. It uses comparators, which don't create light, avoiding light update lag.
Point being, if you're rich and want a mob farm, this is one way of doing it.