I mostly install GNU/Linux to either mess around with or to code on and build Haiku.
For playing around I tend to stick with what get's me up and running the quickest which usually tends to be Ubuntu. I like the mix between the relative stability of Debian and the "bleeding edge" mentality of OpenSuSE.
For coding I stick with a minimal, console-based enviroment I've cobbled together on top of Arch. No huge fan of their packaging system but I like it's bare-bone style and ease of managment.
Edit: I also have three Unix systems gathering dust in a closet, one DEC Alpha PWS500AU running Tru64 and two SGI Indigo2 of which one is running IRIX 6.5 and the other one's for spare parts.