- Akismet API: Automatic Kismet, or Akismet, is a collaborative effort to track and prevent blog comment and trackback spam. They offer a hosted service, an API and also this interesting Spam Zeitgeist stats page with 218,684,318 spams caught so far and 93% of all comments they see are spam.
- Freshbooks API: Nice online invoicing and time tracking service with a web services API. Formerly named 2ndSite, headquartered in Toronto.
- Microsoft adCenter API: While this new service has an API it is currently only offered to a limited number of partners. Some details are available via the links from the profile page here.
- Commission Junction API: "CJ", the well-established online advertising company (est. 1999) with one of the largest affiliate networks, now offers an API. For more see their recent press release.