As far as I know, the only major upgrade that was free was the one that allowed you to go from 10.0 to 10.1. Major upgrades after that were $129.
I got to use the Public Beta (which had the Apple logo in the middle of the menubar and was WAY weird) as well as 10.0 and 10.1. You can definitely see how much Mac OS X had progressed (and at times regressed) since then. Fonts had shadowing on them, which made them a bit harder to read (they don't have shadows anymore). Also, the Finder didn't have the spring-loaded folders when you opened them or when you dragged and held a file over a folder. Lots of the stuff that was standard in OS 9 wasn't included originally in OS X but was included with 10.2, I believe.
Also, anything older than 10.2 was a little sluggish since the code wasn't optimized as much for the hardware.
I got to use the Public Beta (which had the Apple logo in the middle of the menubar and was WAY weird) as well as 10.0 and 10.1. You can definitely see how much Mac OS X had progressed (and at times regressed) since then. Fonts had shadowing on them, which made them a bit harder to read (they don't have shadows anymore). Also, the Finder didn't have the spring-loaded folders when you opened them or when you dragged and held a file over a folder. Lots of the stuff that was standard in OS 9 wasn't included originally in OS X but was included with 10.2, I believe.
Also, anything older than 10.2 was a little sluggish since the code wasn't optimized as much for the hardware.