For those who are working on accessibility using Safari and WebKit, there are some tools available, though as yet, there seem to be fewer than for other browsers.
Apple provides guidance for developers with respect to working with Safari and assuring accessibility. The Safari Developer Library is the best place to start.
The Using ARIA 1.0 and the WebKit Accessibility Node Inspector (Surfin' Safari) post by James Craig provides a few widget examples and demonstrations of how to inspect code written with ARIA in mind.
Finally, the UI Browser is another tool that may be worth considering, depending on your needs. It is not free, and it is not an accessibility testing tool, per se; rather, it helps with assuring accessibility for GUI scripting, used on Mac OSX when creating Apple scripts.