ka-map is an open-source mapping project aimed to providing an easy way to implement online mapping for your own applications/websites (via O'Reilly's Dev Blog). Think GoogleMaps, but open-source, and therefore much more configurable. Also, you won't be tied to Google's beta status and therefore their freedom to change or not document the API. And, unlike Mikel of BrainOff, I prefer using more embedded/transparent web technologies than Flash for maps and the like.

ka-map is built using MapServer. I've tried getting MapServer setup, but haven't finished it yet.

Give it a try at DM Solutions Map. Unfortunately for much of the world, this is Canada only, but still a very good demonstration. There is a US and World versions supposedly in the works.