Mac OS X is Darwin BSD based. FREE to anyone, for PCs, and the FREE Mac version is OpenDarwinBSD based.
Macintosh is worse than Microsoft about hiding the cookie, so that you will have to buy their "upgrades".
So when you buy OS 10.2.x you get that, plus upgrades up to 10.2.9 but, you have to buy 10.3.x and then you get all the 'upgrades' through 10.3.9
Now, If you have bought 10.4.x, you get it all free through 10.4.9.
So, there is actually no 10.2.9 because it stopped at 10.2.8 and the next upgrade was 10.3.0 which went up to 10.3.9, which, again, everyone gets to purchase.
I broke the chain of pain, the expense of upgrades, by getting one of the 30 FREE Linux distros built for Mac 601,603, PowerPC, G3,G4,G5 processors, or by using the OpenDarwinBSD for them.
Yes, there is a Santa Claus, called Free Open Source Software (FOSS). And, Google loves Macs, http://google.com/mac