You missed FreeBSD.

Just for completeness, I don't hate it.

Update: Symbian OS too.

Update^2: and more importantly, OS X and MacOS Classic too, some people might want to vote on those.

Update^3: And BeOS. And Knoppix if you list Linux distros.

Update^4: And Windows 95 OSR2.

Update^n: And Windows Longhorn (we can hate it even before it's released, can't we?)

Update^n: Oh, that's the same as Vista, sorry.