If you have Visual Studio 2005, the free edition of Ruby In Steel gives you 
most of what you want:
- color-coded editing
- multi-level undo/redo
- code folding
- debugging with breakpoints, watch variables, locals, globals for Ruby and 
for Rails
- trace-into/ trace-over
- project management (tree structured projects of files and folders)
- project import
(etc.)

The free edition doesn't have intellisense but this will be added (including 
scope-and-type sensitive auto completion, 'smart indenting', tabbed 
snippets - among many other things) to the commercial edition due out in 
January.

best wishes
Huw Collingbourne

http://www.sapphiresteel.com
Ruby Programming In Visual Studio 2005