I did not try it yet, but it sounds interesting.

From Ruby In Steel IDE FAQ:

- What is Steel?
Steel is the name of a free Ruby programming environment for Visual Studio 2005.

- Will Steel provide support for Linux or OSX?
The Steel IDE will only run on Windows – not OS X or Linux. However, there is nothing preventing you from developing under Windows and running the debugged Ruby scripts under other operating systems.

- What are the requirements to run Steel?
Visual Studio 2005 standard edition or above; Windows XP (service pack 2).

- Will Steel run in Visual Studio Express or Visual Studio 2003?
No. The Steel IDE has been developed exclusively for Visual Studio 2005, standard edition or above.

- Will Steel have debugging and IntelliSense for Ruby?
Yes. We shall be providing debugging and IntelliSense (‘code completion’) support in future releases of Steel.

- Will Steel provide support for Ruby On Rails?
Absolutely. It is our aim to provide full, integrated support for Rails development.

- Will Steel always be free?
There will be two versions of Steel - a standard edition and a professional edition incorporating more advanced features. The standard edition of Ruby In Steel will always be free.