I would like to know of various web applications that deal with users sending their locations, in order to process or provide some service based on that. Preferably websites with APIs, but not limited to those only.
Here are a few examples:
You can upload your entire GPX track and it will let you process those traces to help with mapping.
Live Track 24
Used by paragliders, who send their locations to this site to log their flight path.
Sending your location to Google Latitude will update a map showing your latest location
Check in to a venue based on your lat-long and earn points.