Chain reaction Legos (a Jon and Calvin creation): turn the crank, the wheel spins and activates a lever, which tips the box forward, which causes the car to roll forward and fall off, the car lands on a lever at the end, and a Lego piece is popped into the air.
The hand mixer (from the Lego Crazy Action Contraptions kit). Jon and I picked up this kit for a couple of dollars at the Borders closing sale last fall and have been holding onto for just the right day ever since. I think they finished four of the projects in the book, including a hand mixer, a "squeezeclaw grabber", a "no donkey donkey cart", and I don't really know what else, but I did notice that with each project they spent some time talking about how it could be better, and trying some of their own takes on it, and that's pretty much a dream come true.
Versatility. That's what I find so appealing about Legos.