Ruby doesn't yet have any truly mature Rapid Application Development tools or IDEs. Expect this category to grow.
SpecRubyThis tool assists in creating Tk interfaces.
Ruby/LibGladeThis tool by Avi Bryant assists in creating interfaces with GTK; it supersedes the older Glade/Ruby by Yasushi Shoji.
Gem FinderThis tool by Jim Weirich is a simple graphical class browser, displaying useful information about subclasses, superclasses, methods, and so on.