Dr Ampersand said:
The appendix. So goddam awesome we don't know what it does.
Not true actually. People have known for years that the appendix is a left over organ that used to be for breaking down tough plant matter when we were still hunter gatherers, but it still hasn't phased out of our evolution yet. The reason it becomes inflamed and needs removal is because in some cases small pieces of food will still get sent there, but it no longer has an exit, so it just sits there for an extremely long time, unabsorbable and unused. If enough food is sent to it then that is what causes most appendicitis.
EDIT: Forgot to put down my vote for best body part award! I'm gonna vote for the Eye. Seriously, without it you wouldn't even be reading this thread, or percieving the world as we know it. Without the eye humans would have evolved as cave dwellers that never even encountered the light of day.