I'm getting into Ruby programming at the moment, and would like to ask what your favourite GUI library/tool is?

The gnome2 and KDE ones are good if you're going to be programming for one of those DE's, and also for general gtk2/qt programming.

The default GUI toolkit, RubyTk, is very capable. Tcl/Tk has been criticised as old-fashioned, not providing some of the more complex widgets and not being object-oriented. If you need seamless integration with KDE or GNOME then maybe it is not the right choice but it is simple to use once you get to know it and is extremely configurable. One of the problems with other toolkits in my experience is the lack of support for widgets which Tk supplies and which I have come to rely on over 18 years of GUI programming in a home environment, such as the Listbox and the interactive Canvas. The Tk Canvas can generate Postscript output, invaluable when creating forms for printing. Creating a GUI under RubyTk can be a verbose task, at odds with the Ruby ethos, but those with metaprogramming skills might be able to refactor their code towards elegance and simplicity. I am still on the toe of the learning curve as far as that is concerned.

Geany is quick simple to use and has instant gratification also covers many languages. What it really needs is a quick simple QT or widgets designer in the same style.

If you want a more structured environment Kdevelop Ruby.
Most disliked Eclipse.

Regards
Ray

Have a look at netbeans 6.8