You have the ability to communicate with your GPS to upload/download waypoints/routes/tracks, as well as import .loc (gc.com created only for some reason), .gpx files and .wpt files.  They are then plotted on terraserver provided sat images, and each cache is independently hyperlinked to its respective cache.  From each cache listed, you can 'find' the closest geocaches, which brings up the gc.com page with those caches listed.  Soon, there will be direct PDA interaction.
 
It's free.  You can also download and view over sat images recent earthquakes in any given area terraserver has satellite photos of.  Oh, and it's written by a geocacher.