I haven't met any celebrities myself, but both of my parents have a long time ago. I'll list who they met later.
Here's who I've corresponded with via email though:
The guy who did the music for Seinfeld along with other shows
Someone who was on the Dean Cain era of Ripley's Believe It Or Not
The mother of one of the actors on Home Improvement
One of the actors who played Frederick Crane on Frasier
A couple of radio DJ's
I also made a funny article about some obscure band on a parody website a few years ago, and the drummer of that said band actually responded to the comments himself.