rbenv
rbenv (Simple Ruby Version Management) lets you easily switch between multiple versions of Ruby. It is simple, unobtrusive, and follows the UNIX tradition of single-purpose tools that do one thing well.
Another tool to be used for the same purpose is RVM.
Installation
Plugins
rbenv can be extended via a plugin system, and the rbenv wiki includes a list of useful plugins. The ruby-build plugin is especially useful, as it allows you to install Ruby versions with the rbenv install command.