If you get the following error when trying to run script/console in Ruby on Rails:

Loading development environment.
/usr/lib/ruby/1.8/irb/completion.rb:10:in `require': no such file to load -- readline (LoadError)
    from /usr/lib/ruby/1.8/irb/completion.rb:10
    from /usr/lib/ruby/1.8/irb/init.rb:252:in `require'
    from /usr/lib/ruby/1.8/irb/init.rb:252:in `load_modules'
    from /usr/lib/ruby/1.8/irb/init.rb:250:in `each'
    from /usr/lib/ruby/1.8/irb/init.rb:250:in `load_modules'
    from /usr/lib/ruby/1.8/irb/init.rb:21:in `setup'
    from /usr/lib/ruby/1.8/irb.rb:54:in `start'
    from /usr/bin/irb:13

You need to install the readline-devel package and compile the readline extension:

cd /path/to/ruby_source/ext/readline
ruby extconf.rb
make
make install  [do this as root/admin]