Redhat 5.2 on one.
Redhat 6.1 on one.
Soon as I get it repartitioned, a dual boot XP/RedHat 7.2 on another.
Soon as I get a CD-ROM installed, Caldura 2.3 on another.

Everything but the dual boot system is, of course, a low Pentium (the RedHat 5.2 is even a 486. I'd have an impressive uptime for that one, only a power outage killed that dream. :)