Debian powers our old Sun mailserver that otherwise would be unusable, and keeps an old Powerbook going. As far as I know no other Free OS's current release supports this Sun hardware (Sparc 10 with dual Ross Hypersparc CPUs), and NetBSD is the only other possible option for the "oldworld" G3 Powerbook.
I originally chose Debian on i386 for its stability and security, but it's just as much the great support for non-PC hardware that matters to us now.