GTK+

The GTK+ toolset does what a Graphical ToolKit needs to do and does it to the point and well.

Pros:

• Common use case keyboard navigation works perfectly.
• Common desktop gui software paradigms feel at home here.
• Latest releases include great intricate debuggers
• Current Stable releases allow for a large variety of CSS for styling.
• First-Party documentation is terse and as such, easy to reference.
• Based on a hilighy-bindable framework that provides the same GUI capability across every major programming & scripting language (even PHP, go figure)
• Cross platform, Yes Window, Mac, and naturally Linux

Cons:

• Scenario-specific documentation and help resources are thin, Often Google returns results for the last major version that are at best only semantically compatible with the latest version. I consider this an artifact of a relatively small user base.
• GTK themes on OSX/Win only emulate native themes, and as such can be shipped with any theme desirable, Deluge on Windows is a good example of a GTK using an irregular theme.
• Native GTK File select/save prompts do not feel native to any OS platform.
• Large tables (TreeViews) perform poorly and do not have a native cache-optimization requiring manually created workarounds.

All in all, I'm in love with this stuff. I try really hard to use the latest major release which has not had 3rd party interest matured for it yet, leaving a lot of unknowns. I that is a problem, GTK2's 3rd party interest is absolutely mature and is ripe with one-off resources that can help particular use cases.