Last year I gave a talk on Ruby GUI toolkits, and concluded that none of those I looked at were compellingly slick or mature. There is a new player on the field now (thus certainly not mature, but interesting nonetheless): Shoes, from why the lucky stiff.
Shoes creates native applications with a Web feel. It is not built atop GTK, Qt, etc., but rather directly on the relevant native API (Win32 on Windows, for example). Shoes has a lot of potential, which it can realize only if a community accrues around it, leading to lots of polish and a rich widget set.
One thing that surprised me about the Shoes install it that, as far as I can tell, it includes its own Ruby install; I installed Shoes on a machine with no (obvious) Ruby installation and it worked.
Actually, it does sit atop GTK on GNU/Linux. But on Windows and Mac OS X, it uses the native widget libraries directly. Eclipse uses the same method of implementing widgets, to good effect. Eclipse’s SWT is the only widget library in Java that I have any respect for.
There’s also “monkeybars” http://www.infoq.com/news/2009/01/monkeybars-gui-mvc