This is my cousin’s daughter. The last time I saw her (or my cousin) was 16 years ago, when she was 9 years old. I doubt she has any idea who I am. Does that count as not knowing her personally? 
I worked for Earwolf for a couple of years and can list dozens of cool LA celebs/comedians/musicians that I know second hand. Best on the list would probably be Jon Hamm or Adam Scott.
My wife went to HS and performed in school musicals with comedian Stephen Lynch.
My mom went to high school with former NFL quarterback Clint Didier.
A friend of mine here in Chicago is close friends going way back with Sean Hayes.
An English friend of ours was once a nanny for Rod Stewart.
I taught college courses for a while and David Bowie’s son Duncan Jones (aka Zowie Bowie) was in one of my classes.
Don’t know how I forgot this - my dad and aunt when to school with Gary Hart (ne Hartpence) in Ottawa, KS.
My SO is a hairdresser. In the mid or late 70’s (I think, it’s before I knew him) he was the head (just behind the namesake owner) hairdresser at Suga Salon in NYC. They had some A-list clients at the time, but the only ones I can remember are Candice Bergen and Dorothy Hamill (the skater - Suga designed her signature hairstyle).
This probably doesn’t count for all that much, it’s not really “knowing” someone to cut their hair, especially since he didn’t speak much English at the time. But it’s as close as I’ll ever get.
My brother-in-law worked at a steel mill with Joe Jackson (MJ’s dad) at a steel mill in Gary.
I taught Trisha Yearwood’s niece when she was a 9th grader.
I sent the drummer from the Velvet Underground’s kid to detention.
One of my teachers in high school is the father of former Seahawks QB Rick Mirer.
I know several people who knew Bob Berdella.
Heh. I once lived next door to Bob Trow, a very cool guy. He played BobDog and a few other characters on the show. He had a beautiful wooded lot with some of the old set pieces on display.
My uncle used to cat sit for Andrew McCarthy.
My step daughter shared a table-for-four with Uma Thurman at a Cannes Film Festival dinner, said she was a really smart, genuine person. She reported “If I traveled in her circles, I could see myself hanging out with her”.
Frank Zappa and Robert Crumb were both overnight guests when my wife had a hippie pad.
Lloyd Ogilvie (later US Congressional Chaplain) was once a member of a small group of my sisters admirers during a family camping vacation. My sister’s roommate dated a teammate of Hank Aaron’s in the minor leagues, but that’s more than second hand, I guess. My best friend’s grandfather regularly rented fishing boats to Aaron and his Braves teammates.
My ex-wife’s neice is Olympic silver-medal skater Tanith Belbin (which I think makes me her uncle) – that’s actually first-hand, I was at Tahith’s first birthday party.
I once had lunch with Satchel Paige’s son.
I had a girlfriend once who had been a backup singer for Ray Charles. And dated another who was the sister of NFL wide-receiver Henry Marshall.
My first wife used to hang out with a girl who became the wife of NFL Commissioner Pete Rozelle. (If you have enough wives, you can cover a lot of ground.)
Before I was born, my parents rented the duplex Wolf Blitzer’s family had lived in. My eldest sister had his room. Our families were friendly, but this was way before I was around.
You win the thread; one step removed from Andrew McCarthy’s cat.
My Dad knew a bunch of the Mercury and Apollo project people.
I’ve got some others but the more I think about them, they are pretty tenuous. For example, went to school with a famous person’s kid, who probably met everyone in Hollywood at one time or another. Also worked for a famous rock-n-roll dude, so, same story. But its not like either of these people were especially close, but enough to be on first-name basis. But I couldn’t tell you who the hell famous *they *knew.
Oh yeah, I had the sons of Fugazi’s drummer as students.
I took several creative writing classes from SF writer Sheila Finch. I once went to a Christmas party at her house and Harry Turtledove was there. She also invited David Brin to give a speech to our class.
I worked with a woman whose best friend was a Hollywood hair dresser. Among his clients was Marlee Matlin, whose hair he did before her Oscar win.
One person who worked face to face with Bill Clinton, Al Gore, George Bush, Dick Cheney, Donald Rumsfeld,and Colin Powell.
One musician friend who while he isn’t ‘famous’ has played with a lot of big name musicians.
One dancer friend who knows Michael Flatley.
Styx played at the prom of the high school I almost went to (if I hadn’t moved), ten years before I would have gone there (had I lived there at the time).
Also, my brother got into a bar fight with Jennifer Aniston’s boyfriend. My mother’s mother was George Meany’s goddaughter. My aunt buried Jeff Ruland’s dog.