Ruby stuff for Macs is a ready-made DMG for both Panther and Jaguar with pre-installed Ruby 1.8.2 and various extensions (including Rails and Rubygems!). Unfortunately, the 10.2 DMG is broken - I've contacted the author.
You can of course also compile Ruby yourself, it's not particularly complicated. For OS X, there's also DarwinPorts. However, the port under 10.2 is acting up and cannot be compiled cleanly. Something in the iconv module - syntax error.
Somehow my system doesn't like Ruby today
Update: now the package works.