For those of you who prefer FireFox over Internet Explorer (and who doesn't), I recently came across a pretty cool extension called Minimap. While it has much of the same functionality of Google Maps, it's nice because it's built right into your browser. You can open the map as a separate tab, or as a sidebar.
It also supports a number of file importing and exporting formats, including GPX and KML, which is great for viewing geocaching waypoint and GPS data. While adding KML data to the map (either by importing or drag and drop) seems to work great, I have yet to successfully import a GPX file - and I've tried everything from pocket queries to my latest GPS track data file. But since this is a fairly new program (18 months old), I expect the program developers will have this feature fixed eventually.
The mapping tool also integrates nicely with other "geo-aware" web services such as Flickr, Panoramia, Local search, Google, Yahoo!, Live Local directions, Tagzania, Platial, Geourl and many more.
You can download and install this free program from the Firefox Add-On page.
Check out this short video which describes some of the key features and browser set-up:
While I've only used this add-on for a short time, I already love it. And once I can get the GPX import feature to work properly, it'll likely become part of my standard geocaching software toolbox.