So to answer your question. I use the Google Maps API.
Why?
I think there are a whole lot of good reasons that make Google Maps an extremely valuable mapping solution for web developers. I realise I will probably sound like a hand waving evangelist, but here goes:
- Very well documented API
- Access to a range of excellent tile sets for your maps (topographic, satellite, road maps, terrain).
- Access to the Google Earth 3D plugin with very few changes to a standard Google Maps site.
- Constant upgrades and improvements to the API and it's features. I started working on my Google Maps project a few years ago and in that time I have seen new tilesets, the upgraded GUI controls, mousewheel zoom, Google Earth plugin ... I am sure this list will get longer.
- StreetView. I don't actually use this myself, but Streetview is a fantastic feature that isn't duplicated in the other mapping solutions.
- Extensive set of very useful tools in the API. GLog, distance calculations, GLatLngRegion etc ...
- GeoCoding. I recently dabbled with the GeoCoding API that Google provides and found it was very easy to use and extremely powerful.
- The Google Maps Group - I have asked these guys for a help a number of times and found prompt and accurate responses that rival StackOverflow.
- Ability to overlay tiles over the Google map tiles (GOverlay).
- Ability to implement entirely custom tilsets that leverage the Google Maps scaling/panning framework.
- Custom map themes allows you to integrate the map look and feel with your page color scheme.