Map mashups are so popular and are such a quintessential use of online APIs that a lot of people incorrectly assume that all mashups use maps. It's no wonder. The first real use of the phrase "web mashup" in this new era can be traced back to Paul Rademacher's HousingMaps.com built in early 2005 shortly after the Google Maps service was released (but before Google even had an official API). Paul created a popular service that used real estate data from Craigslist and plotted property listings on Google maps to make something genuinely more useful than the sum of the parts.
Today, just a couple years later, the online mapping space has exploded. Competition is fierce, the stakes are very high, and most of the major online service providers offers a mapping service for users and a mapping API for developers. It's a multi-billion dollar market that overlaps with online search, local advertising and is applicable to vast range of commercial applications.
If you filter the APIs on ProgrammableWeb you can see there are over 50 APIs related to mapping and geo-location. That's a lot of mapping-related APIs and constitutes about 10% of all the APIs listed at ProgrammableWeb. They may not all be what you expect. Here's a breakdown for you:
The ones you expect: the Google Maps API, Yahoo! Maps API, Microsoft Virtual Earth API, AOL MapQuest API (and not to forget GIS encumbent ESRI ArcWeb). Each of these major vendors offer developers a free level of service as well as fee-based commercially licensed options.
Competitors in this category include the GeoIQ API for rich data visualization that builds on top of Google Maps. The HopStop API lets you integrate mass-transit and walking directions into your own website. For "personal geography" and social community mapping there's the Platial API or the WayFaring API. If you want maps that understand neighborhoods you could use the Urban Mapping API. Share your location and discover others nearby with the Plazes API. Get 3D mapping capabilities with the Poly9 FreeEarth API. Or, a bit more unusual is the Where's Tim API that let's you track the location of Tim Hibbard 24x7.
These essential services turn addresses into latitude and longitude: geocoder.us, geocoder.ca, Ontok and Yahoo! Geocoding (note that Google and others now offer geocoding within their own mapping APIs).
This includes APIs like the USGS Elevation Query Service from the Geological Survey returns the elevation in feet or meters for a specific latitude-longitude point. NASA provides mapping images via their satellite image API.
The European mapping leader is the Multimap, down under in Australia and New Zealand there's the Whereis API and ZoomIn API, and in Korea you'll want to use the Naver Maps API. There's also the Nearby.co.uk geocoding API and also for the UK is the iShareMaps On Demand API that geocodes UK addresses to a postcode level.
There are many thousands of maps mashups out there and we're now up to a catalog of 1100+ map mashups listed here.