The core Ruby on Rails development team uses TextMate, a MacOSX (only) programming text editor that is apparently the greatest thing ever. A quick Google search reveals countless blog and forum posts devoted to finding something with similar functionality on Linux and Windows.
I am starting to develop with Ruby on Rails on Ubuntu and three of the suggested alternatives looked attractive to me. If you are planning to develop exclusively in Ruby/Ruby on Rails and want the lightest weight solution that is probably closest to the TextMate experience, Gedit is a good alternative. But if you want a fuller experience, or if you develop in multiple languages, jEdit and Eclipse are (I would argue) the two best options.