“Big Cat” references aside, Mac OS X 10.5 “Leopard” is pretty nice. My favorite feature so far is probably Spaces – though others have pointed out that it’s hardly the 1st implementation of “virtual desktops’ on Mac OS X, and it’s missing some features, it’s (understandably) the best integrated of any implementation so far. And it makes my 17” monitor at work actually useful. My only gripe is that I want/need different desktop backgrounds for each Space so that I can remember where I am.