I'm fond of software I like to call "pervasively programmable", Emacs and the various Smalltalk environments being ur-examples. Every bit of them can be examined, inspected at run time, and changed in the running system as you work. It's the next step beyond open source. (If you compiled Emacs yourself, Emacs will remember where its C source lives and will happily bring that up if you M-. on one of its C primitives.) And I think we should celebrate and encourage software like that.