For me, it'd be a close race between Debian Linux and Solaris (2.6 - 10), with Linux being used that much more.
I use mostly Sparc/UltraSparc-based machines of varying vintage, used mostly for learning and research purposes. My next goal is to fix my A2000 and use it as an Xterminal under Amiwin (for no reason other than "because I can", practicality be damned).
