API Harmony was just added to the list - not just discovering the API but also relating the API to usage, SDKs, discussions, and other relevant information a developer may be looking for.
There are many great options in the list but with the state of industry and the large number of APIs coming out each year Google is still a great place to start!
Curious if folks perform their initial search on any of the sites above or if Google is the one that lands you there?