This is a Mob Fountain following the basic design outlined by zorac in this thread: www.minecraftforum.net/viewtopic.php?f=35&t=83117

Basically mobs float to the top from a spawning room underneath. They pop out the top and then plummet to a burning death. Very entertaining.

The fountain works because water behaves VERY differently when it's adjacent to glass or ice (the fountain wouldn't work if you replaced the glass blocks with dirt for instance).

You can make the fountain as tall as you like and the mobs will float to the top.

There are 4 zombie spawners in a room below ground to supply the fountain.