I had a class with Tiger Woods for a quarter. He only showed up half the time and looked pretty angst-ridden when he did. Shortly after, he dropped out of college to make bazillions of dollars, so I don't think he was angst-ridden anymore. :D
I almost walked into Fred Savage one night. I totally wouldn't have noticed it was him except that my friend pointed it out.
Chelsea Clinton was also there at the same time I was but I never saw her because it's a huge, sprawling campus and the various departments are pretty separate. My dad was quite anxious that I'd never seen her and he told me I should find out what dorm she was in and like, hang out there, eat lunch or whatever, and I asked, "So you want me to stalk her?" and he answered, "Yes."
He was serious.
I already know certainthings's Nelly Furtado story, how about everyone else? :)