I don't know anyone per se, but my wife was Bryce Harper's (and Bryce's brother) teacher in elementary school. Although we no longer live in Vegas, she still knows his parents fairly well. Bryce sent her a signed copy of the Sports Illustrated cover he did last year when he was being hailed the Prodigy. We are also good friends with his high school coach. I'm hoping Bryce will give my boys some baseball tips in a few years.
I have met a ton of UFC fighters, and some other celebs at UFC fights (Victorino from the Phillies, etc.). I have a good friend who works for UFC. As an aside, I have to say mixed martial art fighters are the nicest and classiest group of athletes that I have ever been around. To a man, everyone I have ever met has been very down-to-earth and extremely respectful to each and every fan.
In my younger years I once got kicked out of a bar in Newport Beach, CA with the ultimate bad-boy athlete Dennis Rodman. He and his wife were having problems that night, and for reasons that escape me they decided to take their feud to this particular Irish bar. My female friend, in a drunken stupor, decided to play marriage counselor with his wife (at that time) while the rest of us continued to drink. He eventually got shut off. I, of course, made the poor decision to continue buying him drinks until the bartender caught wind of what we were doing.
I've played a pick-up game or two with some eventual NBA players during my time in graduate school at UNLV (Marcus Banks, Lou Amundson, ect.).
That's about it for athletes.