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.