alt Hacker NewsNetBSD was my favourite playing around OS from the late 1990s to about 2010. Worked on any old crap I had lying around which was mostly thoroughly obsolete SPARC and x86 kit. Did the job. Never went wrong. Good documentation built in. Netbooted on anything.
No idea where it is now but this post has inspired me to have a look.
I was a fan of FreeBSD over NetBSD; far more driver compatibility in the 1990s. It was nice to play with. Way too hard to use for much (well, online) now, due to the shrinking number of software packages (eg browsers) these days. I still remember how good it used to feel installing NetBSD on (now long long dead) laptop in the early/mid-90s.