Wikimedia Commons API

Usage : commonsapi.php?image=IMAGENAME
IMAGENAME must not have an Image: prefix
Optional parameters:

&thumbwidth=XX returns a URL to a thumbnail maximal XX pixel wide
&thumbheight=YY returns a URL to a thumbnail maximal YY pixel high (requires thumbwidth)
&languages=en|de|default returns only the languages for the listed codes; separate with "|" (default: all languages)
&forcehtml converts wiki text into HTML (slower, but gets rid of annoying wiki markup)
&versions adds information about all former versions of the file
&meta adds meta information that is stored with the file (e.g., EXIF data)

Examples :

Sa-warthog.jpg with thumbnail, version, and meta information
Yarra_Panorama.jpg (contains geocoding, which resolves to a <location> tag)
Multiple images and one (deliberate) error, plus meta information

Note: All returned attributes and texts are entity-encoded ("'<>& are replaced with XML entities).

(You want to help making this API better? Data source for license information is here. Bugs and feature requests go to this guy.)