Disable Support Site Help Searches in Leopard
When you use the search box in the OS X Help system (in OS X 10.5, at least) it will, by default, search both the local help and Apple's Support site for topics that match your query. If you've got a slow or not-always-on Internet connection, or if you'd just rather not have your search terms sent to Apple, previous versions of the Help system made it easy to disable these searches via the Help system's preferences. In 10.5, though, the Help system was revamped, and it's no longer a standalone application--which means it doesn't have any preferences to set.
To disable searching Apple's Support site in 10.5, you need to just look in a not-so-obvious location. At the left edge of the search box in Help is a small magnifying glass icon with an (easily-overlooked) disclosure triangle. Click the magnifying glass, and a small menu appears with three options. The first two let you choose between searching Help only within the calling application, or across all applications. The last entry, Include Product Support Searches, is the one that controls the Support site searches. Remove the check mark, and your Help system searches will no longer return results from Apple's Support site. The setting here is global, too--you don't need to uncheck the setting in each application from which you open Help.
Thanks to Mac OS X Hints reader dilvish1984 for pointing out this well-hidden setting.