I still use Mac OS 9 off and on again, simply because it's the latest OS that runs a program originally written for System 6 (I think, but it was before I was born so I'm not sure). In the early 2000s there were plenty of people dual booting since some of the dome iMacs (only the 17" model I think) came with it installed and ready to go. Others ran Classic environment from within OS X 10.1 or 10.2, but there were odd crashes and mysterious memory and disk errors that would come up now and again. Similar problems with running it in SheepShaver/BasiliskII though I haven't given up on it yet since I would like to have it available on a modern laptop. But back to the OS... what I find most interesting is how much stuff already existed but got cut for whatever reason during the transition to OS X. WindowShade, colored Finder labels (made their way back in 10.3 or 10.4), Voiceprint (heh), etc.
If you want a quick dose of nostalgia, open Stickies. Yeah, it's in your Applications folder at least up to 10.9.