Rob Griffiths, Macworld senior editor and creator of MacOSXHints.com:
Butler ( )
This powerful tool does everything for me—launches programs, controls iTunes, creates navigable pop-up folders, runs AppleScripts, and even handles basic macros. It’s my one can’t-work-without-it utility.
Chax ( )
I have dozens of open chat windows most of the time, so Chax brings me order by adding a tabbed window to iChat. This essential iChat add-on also has built-in log and activity viewers.
Google Earth ( )
I used to take out my bike or car to measure routes that I like to run. Now I just use Google Earth’s path tool to get an accurate measurement without leaving the house. When I’m done, I can copy and paste the map of the route, and keep it for future use. (Oh, yeah—it’s also a great way to explore the planet.)
If you use keywords in iPhoto (and you should), Keyword Assistant makes it much easier to add, delete, and work with your keywords. It’s an essential add-on for high-volume users of keywords in iPhoto.