Geotagging photos on the mac
With a couple of bits of free software you can simply geotag (add longitude/latitude to a photos EXIF data) your pictures. These can then be shown on flickr's map, google maps etc.
A really handy command line utility and graphic GUI for translating GPS formats. It can grab from file or from various devices and can convert between many different formats.
A really nice utility that can take a GPS XML (gpx) track and a set of photos. It can then tag the photos based on the track.