The ancient (read 90s) distros are a lot of fun to play with. Redhat still have a lot of their legacy distributions available and they are installable on vmware. You can learn a heck of a lot with really early distributions. I remember how complicated they seemed at the time but compared to nowadays it's pretty easy to have all the system components in your head at any one time.

I spent a week or so upgrading component by component redhat 5.0 (hurricane) to be able to run a more modern version of glibc learning a lot about the c runtime stack under linux in the process.