Google Earth
Google Earth is an amazing program that was originally developed by people at a company called Keyhole. Keyhole was later bought by Google and the program was renamed to Google Earth and released for free (thank you, Google!). If you haven't tried this program, please do. It is now available for Windows, Mac and Linux so chances are it can run on the machine where you are reading this. You can download the program here. What it does is to give you a virtual globe in you computer, combining satelite imagery, maps and terrain data for the entire earth so you can fly around and zoom in and out to any place you want.
Google Earth
Hacking Google Earth
One of the most interesting feaures of Google Earth is the ability for users to add more information to it. Google Earth supports a special mark up language called KML (Keyhole Markup Language) that allows users to add a wealth of stuff. You can have it draw lines and placemarks with comments, overlay image layers on top of the default imagery, and much much more. You can read a lot more about KML here, and you can find thousands of examples of data that others have added on the internet. Two good places to look is the official Google Earth community at bbs.keyhole.com and the site Google Earth Hacks.Apart from KML, Google Earth can import a lot of other formats as well, like images of various formats, GPS data files and even simple text file if they have a resonably strucured content, such as comma separatef fields. It can also directly import waypoins, tacks and routes from GPS devices like Garmin and Magellan. So you can take your GPS with you when you are out and about and then plug it into Google Earth and relive your travel. Anything imported in the diffent formats can then be exported in the form of an KML file.