Updated at 11 p.m. PDT with link to troubleshooting support.
Several security issues are addressed in the update, including issues related to Apache Web server, PHP, CoreGraphics, and Safari. The update patches three separate vulnerabilities in Apple Type Services, QuickDraw Manager, and WebKit, all of which could allow for arbitrary code execution.
Among the hardware support enhancements is additional RAW image support for third-party digital cameras--a format that allows for retention of more image information than JPEGs. The update also improves performance of video playback and cursor movements for recent Macs with Nvidia graphics components and resolves an issue with Dvorak keyboard layout in Mac OS X 10.5.6.
OS X 10.5.7 includes patches for issues related to network stability and performance, as well as issues related to logging in to Gmail and synching contacts on Yahoo. The update also includes a patch to improve the accuracy of several Dashboard widgets, including Unit Converter, Stocks, Weather, and Movies.
Improvements to iCal include improved reliability with CalDav as well as automatic syncing with MobileMe. Apple has also improved its Parental Controls and applications restrictions.
The update, which is recommended for all users running Mac OS X Leopard, is available through a standalone installer or Software Update.
Some users have complained about installation issues related to the update. For troubleshooting tips, see MacFixIt.