Automatically Geotag your pictures

Wikipedia: “GeoTagging, also known as GeoCoding, is the process of adding geographical identification metadata to various media such as websites, RSS feeds or images. This data usually consists of latitude longitude coordinates, though it can also include altitude and placenames.” Lots of people have started GeoTagging their images on Flickr and then looking at them on Google Maps and Google Earth. As of this post there are about 58 000 images on Flickr that have been “geotagged.” I thought this was pretty cool so I wanted to figure out how do it automagically. How can I geotag my images without having to do it manually?