There's always the ones where people claim they, or someone close to them, got in a car accident. Those are usually less fun...
One of my friends who has been strictly Atheist for at least as long as I've known him, proclaimed one April 1st that he believes in God and was wrong all along. THAT was a pretty controversial April Fool's Day. He posted it as his Facebook status, and I think by the end of the day there were at least 100 comments, first congratulating/chiding him, and then breaking out into an all-out rage-fest.
I'd never have the guts to do something like that.

Also, I can never convince my boyfriend that I'm pregnant on April 1st. First of all, I'm a horrible actress, but more than that, for some weird reason, my time of the month seems to always conveniently land on the 1st.
Oh well, such is life.