Interested in seeing which Mashup APIs are popular? This is the topic in this recent discussion post... interesting! This article from Apex devnet provides some insight on developing a mashup using the slaesforce.com platform... The second article in this series presents several concrete techniques for building mashups. By the end of the second article you will be in a position to comfortably create your own mashups on the Salesforce platform. The article looks closely at the Gogole maps API, using AJAX, and other APIs... http://wiki.apexdevnet.com/index.php/Mashups:_The_What_and_Why Of interest, some of the most popular APIs for mashups according to programmable web include:
Google maps 50%
Virtual Earth 3%
Looking for data or tools for your mashup? Try some of the following:
Yahoo! Geocoding service - http://developer.yahoo.net/maps/rest/V1/geocode.html
USGS Geonames - http://www.geonames.org/export/
USGS Elevation Service - http://gisdata.usgs.gov/XMLWebServices/TNM_Elevation_Service.php
So what kind of mashup are HOT besides mapping? According to programmable web, video mashups, social mashups and photo mashups are leading the pack lately!