The Google Operating System blog points out 10 great uses for Google Desktop, including file versioning, recovering deleted documents, offline Gmail search, and application launching.
Personally I’m a big fan of Google Desktop. Control, Control has become my regular Windows keyboard shortcut twitch, and I can’t count how many times I’ve found files and emails I’d forgotten about using GD’s advanced search operators. Not to mention the GD Timeline, a neat way to see exactly what files you were working on a given date. A Mac version would be nice, though, Google. See also: Master Google Desktop Search.