Ruby One-Click Installer
... This package is the most simple way to equip your pre-Leopard Mac Apple OS X System with Ruby - similar to the Windows Ruby One-Click Installer. It replaces the broken Readline library, updates to a current version of SQLite3 and prepares your OS X for Rails, which needs at least Ruby 1.8.4 to run. The current Ruby Version is 1.8.6 (1.8.5 is recommended for Rails) and Rubygems 0.9.4.
Note: This is NOT needed for Mac with Leopard. Leopard already inlcudes a recent version of Ruby and now includes Rails.