A nice comparison sheet of the various Ruby posted over at O'Reilly Ruby.
This sheet is actually from another blog: "The Nameless One"
It is funny, to me at least, is that the O'Reilly post is written by Curt Hibbs. Curt is one of the principle developers on the FreeRIDE Ruby IDE; he mentions that in his post. The interesting part is that the comparison did not include FreeRIDE. This is for a good reason. FreeRIDE is a joke. FreeRIDE really stands to make Ruby look bad to people being introduced to it for the first time. FreeRIDE is unstable at best, bordering on unusable.
A first timer on a Windows box is probably going to download the one-click installer to install Ruby. This installer comes with FreeRIDE and will put shortcuts out for it. The afore mentioned first-timer will launch it and try to play with it. It will be slow, clunky and featureless; then it will crash. With an immediate "wow, well screw that" from the newbie.
It sort of reminds me of much of the worlds view of Java due to all the shitty Applets and Swing/Awt apps out there.
If you're looking for a good 'free' rails IDE check out RadRails, if you're a Java guy with IntelliJ installed, definitely check out the Rails plugin.