The Freesound API allows developers to programatically browse, search, and retrieve information about Freesound users, packs, and the sounds themselves of course. Using the Freesound API one can find similar sounds to a given target (based on content analysis) and retrieve automatically extracted features from audio files, as well as perform advanced queries combining content analysis features and other metadata including Audio Commons descriptors. Freesound API also allows to upload, comment, rate and bookmark sounds.
Have a look at the Freesound API documentation for more information about the Freesound API and how it works. Also checkout the Freesound Labs entries tagged with apps for some examples of applications that already integrate Freesound content through the Freesound API.
Freesound was started in 2005, and is still developed and maintained at the Music Technology Group of Univeritat Pompeu Fabra, Barcelona, Spain. More information about Freesound can be found in this entry.