[Aug 22, 2009; Outdoor]

Using Google Maps overlays to visualize GPS recording is a pretty obvious idea, but I'm too lazy to look at their API. A quick search revealed a site which offers this functionality:
It does not even require registration. A unique (secret) name for the uploaded track is enough to be able to edit or delete a track later on. Modifying the track after the upload turned out to be pretty easy. And this is what the result looks like: (last week's MTB tour)

It not only supports Google Maps, but also provides access to the mapping services of Open Street Map, Microsoft and Yahoo.

Detailed view (

Sebastian (Sun, 23 Aug 2009):
You can achieve similar results directly with Google Maps. If you are registered, you can create your own map and add tracks to it. You can also embed those maps in your website.