Leopard's Ruby
Ruby is now an official part of Leopard. Well, it seems more complicated than that. It seems that there are difficulties updating the gems, since the gem system installed by default in Leopard is 0.94 (gem -v) and gems isn't Leopard-aware until 0.95. It is now at 1.0. Lots of workarounds have been suggested.
I'm not going to bother with all that right now. The defaults should be sufficient for my immediate purposes.