My mom went into labor with one of my sisters at his first birthday party.
This same sister (also a SDMB member) is on a CD of a rather well known musician.
Funny enough, most of the name-dropping I could do is post-famous, but the few pre-famous include:
Marilyn Manson – we saw him at a little club in Nashville when he was literally a nobody, we just sat and laughed at him then went and got high with him later. It was before he had any kind of record deal, IIRC.
My sister, the porn star. Uh, I don’t know if being in a boatload of porn movies makes you exactly “famous” but she seems to think so, so there is that.
I partied with Perry Ferrell when Jane’s Addiction was still an opening act, but I knew then that he was going to be huge – he is so amazingly talented.
Pretty sure that’s the extent of my pre-famous encounters, although I think I may have forgotten a few.
My husband met Barack Obama at a small Chicago fundraising event when he was running for the Illinois Senate seat, about 6 months before he gave The Speech. He said Obama was impressive as all get out and would someday be Big.
I used to go to the Little 5 Points Pub in Atlanta, where I met a couple of young female musicians who had a lot of talent. Their gigs were always very well attended, and I have an autographed copy of their first album (on vinyl!) they gave me.
Flash forward a few years, I’m married and living in Hawai’i. Who should come on the radio but the Indigo Girls!
When one my past bosses attended the University of Missouri Columbia one of his best friends was dormmates with Brad Pitt. He met him several times but Brad never became a part of their group of friends.
I occasionally hung out with Jen from that show “Ten Years Younger” back in DC in the mid-90s. Er - maybe, I’m pretty sure it was her. Knew her as Little-Jen, in contrast to her SO Big Jen.
Oh, crap, I forgot a big one! I ate lunch and talked for about an hour with former U.S. Attorney General Alberto Gonzales, back when he had a modest position as a Texas Supreme Court Justice, appointed by Governor Bush. I was just about to start law school and he talked to me about it and a subsequent career in law. He was really, really nice. It made me a little sad to see his recent downfall.
On a slightly more humorous note, he came to speak at my law school to a small group of people a year or so later. I thought about going up to say hi afterwards, but decided against it. When I told my dad later, he said “Hell, you should have! He would have remembered you, politicians have to gladhand and remember people’s names all the time! I’m sure he’s got a great memory for people!” 
You just reminded me of a funny (to me, anyway) story.
The sister of a friend of mine went to Harvard and is extremely oblivious to pop culture. One day they (friend and friend’s sister) were watching TV together and Good Will Hunting came on. Suddenly sister exclaims “Hey! That’s Matt Damon!”
Friend says to sister: “Um…yeah.”
Sister says, completely oblivious to the fact that Matt Damon had been a famous actor and screenwriter for years already: “No, seriously, that’s his name…he lived across the hall from me at Harvard!”
Wow - small world 
…especially considering how tiny the towns in that area are (assuming that’s where this happened).
(and welcome to the board!)
Growing up, we were family friends with the Reynolds, who’s son Kevin went on the direct The Beast, Fandango, etc.
John Hancock (Midnight in the Garden of G&E, The Alamo) and I were fast friends from rush.
Dated Robyn Bernard in college. Then Crystal was just her cute little sis.
Know a bunch of guys that went on to NFL fame… Walter Abercrombie, Vann McElroy, Ray Berry, Mike Singletary, Alfred Anderson, Keith Bishop, etc.
I went to high school with one of the (future) space shuttle astronauts. She was one of our valedictorians (no big surprise there).
My future son-in-law went to high school with Joey Fatone. My daughter was all excited when she was introduced to him at a party a few months back. But I guess that doesn’t count against the OP because I didn’t meet him.
As for pre-semi-famous, I overlapped one year in high school with a girl who became a Rockette and one of Dean Martin’s Golddiggers, then she did a few bit parts on TV. Except I never really met her, either. I knew who she was - everyone knew who she was in that school. I saw where she created her own IMDB page, but I doubt that anyone would recognize her name.
I went to high school with Jon Nolan who co-wrote Memento, The Prestige and the new Batman movie with his brother Chris Nolan.
I almost did, but then they ended up not being famous.
I graduated high school with J.J. Redick, who currently plays for some NBA team or another. Don’t care; I hate basketball and he was a pompous dick.
I mostly just chose to ignore his existence, but I’ll always remember one fateful day in sophomore year gym class. We were playing round robin 1-on-1, and when it was my friend Deric’s turn to play J.J., he decided to subscribe to the theory of “if you can’t beat 'em, wait for 'em to dunk and hang on the rim like the pompous dick they are, steal their gym shorts, run out of the building and go home.”
You won’t read about THAT in the sports section.
I went to school with Peter Andre (back when his name was Peter Andrea), before he could sing. Nowadays his only claim to fame is being married to Jordan (aka Katie Price).
I know all the guys from The Used pretty well. I came up through the same central Utah local band scene as they did, in their many incarnations, and probably shared 20 bills with them over four or five years. Their first drummer, Branden, actually filled in on drums for my band once on a road gig at some little college in Manti, Utah. He did so, knowing full well we would be lucky to get gas-and-food money out of it. Totally nice guy.
Branden is now the drummer for Rancid, which gives me, like two degrees of separation from one of my favorite bands.
\m/:mad:\m/ <— I rawk by association.
As I am sure you already knew, those two sisters were famous in certain circles when they were little girls. Their father was an evangelical minister who was closely aligned with Jerry Falwell in the 70’s. I was a DJ at a college radio station in the 80’s and we had a Jerry Falwell album that had music and preaching on it. There were two songs on it by the young Benard girls that we would (ironically) play all the time. I wonder how Robyn’s daddy felt about her dating the likes of you. 
I was good friends with Crispin Glover from 8th to 11th grade. His dad, Bruce Glover, was a respected working actor and Crispin wanted to get into the business for as long as I knew him. His Dad knew what went down with child actors and wouldn’t allow him to get started until he was 16. That weird guy act that he puts on isn’t much of an act.
In the late 80’s and the aforementioned radio station, this obnoxious lady came in, uninvited, with a young guitar player. She was his agent. She “asked” us to interview him and wouldn’t take no for an answer. She made us promise to play the interview that evening on the air before she would leave. The guitar player was kind of embarrassed and sheepish the whole time. He was more of a heavy metal type and we were an alternative station.
As soon as they left, we bulk erased the interview and mocked them for weeks. We were way the hell too cool for him. The guitar player was Steve Vai who had gone on to some success and is very well respected.
Not me, but my dad apparently went to university with Ivan Reitman (who did the Ghostbusters movies).
The late 80’s? You guys missed out! Vai had enormous cred even then, he’d played and recorded with Frank Zappa, David Lee Roth, and even Public Image Limited!