I attempted to install ruby-debug gem but it failed with a message:
ERROR: Error installing ruby-debug:
ERROR: Failed to build gem native extension.
/System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/bin/ruby extconf.rb install ruby-debug can't find header files for ruby.
This was because the installed Mac OS X Xcode Tools was old, specifically, GCC was old. I had to install Xcode Tools 3.0, which is actually shipped with Mac OS X 10.5 Leopard DVD. You can also download it.