I'm seeing all these people counting *versions* as OSes. Feh. I can get to 11 easy, and I'm probably missing some:

1. Mac System/MacOS (6.0 through 10.2)
2. Windows (3.1 through XP)
3. Linux (since '93)
4. MSDOS (3 through 6)
5. Apple DOS (3?)
6. Apple ProDOS (for ][e and GS)
7. PalmOS
8. SunOS/Solaris
9. IRIX
10. Amiga OS
11. C64 OS

Also, I used a bunch of computers like the TRS/80 and the Timex/Sinclair 1000 when I was really little in the early 80s, but I don't even remember what-all there was---my parents were both teachers and were able to get loaner computers over the summer, a different kind each year. Fun!

Check out that perfect bell curve, though. :)