This is a cool little trap that me and some friends came up with while messing around with TNT and redstone on a big platform in the sky. The reason we called it the 'Roof Lever Trap' is because it utilizes the ability to place levers on the bottom side of blocks, which was introduced in Minecraft 1.3.1.
It's fairly simple. When you remove a block, it sets off the TNT below. The trap is extremely compact, the smallest version being 3x3x3, and it can be modified for other purposes.
I think its pretty cool because no one ever suspects to get blown up from removing a block.