Our API directory now includes 54 file sharing APIs. The newest is the Gigatribe API. The most popular, in terms of mashups, is the FilesTube API. We list 1 FilesTube mashups. Below you'll find some more stats from the directory, including the entire list of file sharing APIs.
Stack Overflow, the very successful question and answer site for programmers, have now released their own API into public beta. The API actually works with all of the Stack Exchange sites, including serverfault.com and superuser.com, with more sites possibly launching in the future. The API is supported by the site http://stackapps.com.