These are people with whom I interacted, not just saw:
Gilberto Gil - passing in a hallway I saw him, stopped and said I enjoyed his music
Peter Buck (REM) - brushed past me carrying his guitar, his jacket was velour
David Byrne - it seems like I help him every year
David Cross - patient with rabid fanboys which seem to cluster about him everywhere he goes*
Zach Galifianakis - sweet man, much less neurotic and volcanic than on stage
Jonathan Demme - I helped him check in and get his badge, very nice fellow
Dennis Miller - low-profile with cap pulled down low, checking in with his family, I'm really not a fan of him anymore due to his politics. Glad I no longer look like him.
Also, I gave Chris Gore (Film Threat) his SXSW badge. When I called out his name, someone down the hall shouted, "Chris Gore sucks!" and they were serious. Mr. Gore and I both laughed at the random hate.
This is sad, but I can't think of any "famous" women with whom I came in contact.
* Funny story. Several years ago I was working behind the registration desks when Carson Daly came through. After getting his badge, he continued to loiter in the area clearly wanting people to notice him. My fellow volunteers and I considered it haughty, then just pitiful when it became apparent that no one cared. Then David Cross came through and there was a visible and audible tumult. I didn't, but wanted to go up to Daly and say, "The only reason that no one is talking to you is that you are a giant tool."