I went to look at the list of API providers that Zapier supports and found that I hadn’t heard of many of them. So I did a quick exercise and did a very, very quick visit to each of the service providers. I captured the URL and a brief description of the services, and tried to categorize them by my own notion of what they do. I made a spreadsheet to capture this info. If you have any suggested changes, please let me know (or if you think I should use a different format, etc.). For what it’s worth, the file is in my public documentation repository on Github, so feel free to fork away, etc.
A list of API's supported by Zapier. Taken from their status page and then did a very brief lookup to find out the URL and create a brief description of the service: