My partner Scott is a big prankster (as you may have guessed). He pretty much lives for April Fools' Day. When I got up this morning, he had switched the right- and left- button functions on my mouse, and switched my keyboard to Dvorak (which makes everything you type come out oh so very very wrong). He put a big rubber cat (one of our Halloween decorations) in the refrigerator. He asked me what was all over my feet (yes, I looked down). He put a Mrs. Miller CD in my car stereo so it would play when I started the car, rather than my usual NPR station. He put together a "Microwave Use Tracking Form" to put up at work, asking people to log what they heated, when, and for how long (he worked today - I can't wait to hear if anyone actually logged their microwave usage).
I had planned on doing the mouse-button thing to him, but he beat me to it. I still did it today - although I'm sure he's expecting it when he gets home. I wish my mind was as creative as his.
Since I listen to NPR a lot, I love their April Fools' Day posts. This is my favorite, from last year - about how maple trees in New England explode when they're not tapped. It's so deadpan, you think it's a real news story at first. And here's this year's story - not as good (in my opinion) but still pretty funny.