SVG

Scalable Vector Graphics (SVG) SVG is a language for describing two-dimensional graphics and graphical applications in XML. It supports placing metadata for the whole document and per object.

Implementations

Inkscape, an open source drawing tool
Open Clip Art Library, a public domain collection of user-submitted clip art
SVG::Metadata by Bryce Harrington is a PERL module for including metadata inside an SVG (used by Open Clip Art Library.

Examples

Here is the code that is in Inkscape's SVN repository for making the License and Metadata Dialogs possible. Please use them as examples for coding your own metadata and license selectors for use in SVG:
Here is the discussion page at the Open Clip Art Library wiki about SVG Metadata.