Oh the possibilities.
I voted 3 since what I have now are:
WinNT 4.0 SP4
Mandrake Linux 8.1
But on my shelf collecting dust somewhere are:
DOS 6.22/WFW 3.11
OS/2 Warp 3
RH 5.2 all the way up to 6.2
Mandrake 7.0 all the way up to 8.1
Caldera something or rather
Corel Linux 2nd edition or something like that
and who know what else.
I've got just enough free space on my HD to actually put several of these others on as well... Oh to have more time in a day.
Alas some day maybe.