> portable GUI (MS Windows, Linux).

There're at least two options: GTK
(http://www.ruby-lang.org/en/raa-list.rhtml?name=Ruby%2FGTK) and Tk (shipped
in standard Ruby distribution ext/tk*). Both work on both platforms, are
well known and widely used.

My guess is that Tk support is more mature, and it's easier to find related
documentation (like the book about Perl/Tk). OTOH, I like GTK more.

wxRuby would be warmly welcomed.

	- Aleksi