I just discovered RVM, a tool for managing multiple versions of ruby on the same system. This is really useful on my MacBook Pro, since the built-in Ruby “has problems”. Such as having gems in weird locations which makes updating them difficult. Plus the fact that the OS X updater may clobber your changes without warning.
Setting it up is pretty easy and the versions of ruby installed are all yours, installed into your environment.
This is doubly useful for me, since I need to work with multiple versions of ruby (1.8.X and 1.9.X).