Maperture is a nifty little plugin for Aperture that lets you geotag photos in your library. We've covered quite a few different geotagging apps for the Mac, but this is the first one that works so well with Aperture as a plugin.
Basically, Maperture integrates an interface to Google Maps which allows you to easily place your pictures on a map, saving the location information as metadata in the photos. Since it's a front end for Google Maps, you get its built-in features like the satellite and hybrid views, etc. In addition, Maperture will automatically place images already containing location information on the map for you. Now the only problem is remembering exactly where you were when you took the picture!
Maperture is a free download from Übermind and requires Aperture 2.