Microsoft is one of the first--and certainly the largest of--customers of Wolfram Alpha's commercial API (our Wolfram|Alpha API profile). For math and nutritional searches, Microsoft's Bing now uses Wolfram results.
Our API directory now includes 45 SEO APIs. The newest is the SEOlytics API. The most popular, in terms of mashups, is the Google AdSense API. We list 77 Google AdSense mashups. Below you'll find some more stats from the directory, including the entire list of SEO APIs.