Some Ruby programmers like to use simple text editors and run programs from the command line; others like a fully integrated IDE with built-in debugging. Here are some possibilities.

3rdRail

http://www.embarcadero.com/products/3rdrail/

This is a commercial Rails-centric IDE for Eclipse. It currently supports Rails 1.x and 2.x only.

Aptana RadRails

http://www.aptana.com/products/radrails/

This is a free Rails-centric IDE for Eclipse.

NetBeans

http://www.netbeans.org/ruby/

This is a free Ruby IDE for NetBeans. The future development of Ruby support in NetBeans has now been discontinued.

Ruby In Steel

http://www.sapphiresteel.com/

This is a commercial Ruby and Rails IDE for Visual Studio.

RubyMine

http://www.jetbrains.com/ruby/

This is a commercial Ruby IDE with emphasis on Rails.

TextMate

http://www.macromates.com/

This is a Ruby editor for Mac OS X.