Geotagger is a droplet for inserting GPS coordinates into your photos.
Simply position Google Earth (version 4 only) over the location you wish to insert into your images, then drag your images (jpegs only) onto the Geotagger icon. Once the images are tagged the information stays with them so you can import them to iPhoto, upload them to Flickr or Panoramio or any number of other things. If you choose iPhoto you may want to check out a plugin I wrote that will export your photos to a KMZ file that you can open in Google Earth and see your photos on the globe.
You can drag images from prettymuch anywhere. But if you drag them from iPhoto it won't show the changes have taken place. The data has been saved to the file but iPhoto has cached the Exif information and doesn't check the image for updates. I recommend tagging images before importing them to iPhoto.