[Hampshire] Adding XML metadata to pictures

Hi,

On facebook one can highlight an area of a picture and add a tag to
it. I.e. Highlight a persons head and add their name.
Is there any open source tool that could do this to my pictures on my hard disk.
I am looking for a tool that would have a file, e.g. "picture.jpg" and
creates a file "picture.jpg.xml" that will contain the metadata
associated with the file.

I could then do my own indexing, and display all pictures with, for
example, friend X in them.

Does anyone know of such a tool?

Kind Regards

James