This may sound a little 'out in left field' but some folks actually uses a web browser for a lot of GUI tasks, especially with scripting-centric languages such as perl.

For Windows, you will find people using Internet Explorer with the "HTA" functionality. For cutting edge folks, you will see some XUL with Mozilla Fire(bird|fox|insert_animal_here).

The hassles are: not very many people use this approach; perl does not have great support for XPCOM (which is analogous to COM objects, but for Mozilla); performance problems; interop problems; security restrictions;

The benefits are: if you know how to do HTML+DHTML+Scripting, you already know how to build a GUI; easily plug in stuff like FLASH, SVG, audio; custom tags; etc etc, anything that fits into a web page you can put in your GUI; development tools are plentiful; pretty-looking output is easy for those with good design sense. Mozilla is constantly improving and has great UI controls ... see XUL.