PreFab UI Browser is a scripter's tool. You don't need it unless you need it, but if you need it, it's exactly what you need. As I explained in TidBITS four years ago ("Scripting the Unscriptable in Mac OS X", 2003-03-10), as well as in my AppleScript book, when you need to write a script that clicks buttons, reads scrolling lists, and chooses from menus, the Accessibility API and System Events are the only way to go; and UI Browser is the best way to figure out how to use them. UI Browser enables you to inspect an application's interface in terms of the Accessibility API, and generates the AppleScript commands you'll use to tell System Events how to interact with that interface.
UI Browser version 2.0 has recently been released. Aside from its new universal binary incarnation and some significant bug fixes, most of this version's visible improvements are small changes to the interface, but they add up to a lot. Most notably, the new "screen reader" responds in real time as you pass the mouse over an application's interface items. UI Browser is now also savvy about interface items that have recently appeared in the Cocoa repertory (such as date pickers), and has some added capabilities aimed at developers who want to test their application's accessibility conformance.
PreFab UI Browser costs $55; the upgrade from an earlier version is $10, or free if you purchased after the start of 2006. It requires Mac OS X 10.4 Tiger, and a full-featured 30-day demo is available as a 1.2 MB download.