Does an (experimental) class browser exist for Ruby?
Check out the maglev/webtools project on github, as well as the rubymirrors gem. It already provides a class browser and workspace for multiple Ruby implementations, and a graphical debugger works on MagLev as well (not so much on MRI).
You can have a look at the Maglev Database Explorer [1, 2] as well.
[1] Video: http://www.youtube.com/watch?v=27mS1BNP7wQ
[2] Gem: https://github.com/matthias-springer/maglev-database-explorer-gem