Amon Tobin after his gig down in London. Pretty cool, but had some mates he wanted to spend time with, which was understandable.
Before that, Jonathan Coulton, when he played here in Edinburgh. It was a fairly small, personal venue, so we had plenty of time to chat with him, pose for photos, etc. We even pointed out how his t-shirt sweat looked like pikachu. Would love to get him back again, Paul and Storm too.
I met both Martin Rees and Robert Winston when I presented a school art project at the Royal Institute in some kind of national competition. Admittedly, they're scientists, not celebrities, but it still makes me squee.
Prior to that, Lynne Reid Banks (author of the Indian in the Cupboard) gave some prizes at my school- I'd won a maths prize. She wasn't very impressed with my book choice though (The Klingon Dictionary). I don't think many people were.... I thought it best to not ask for an autograph.