Exempi bundles the above two tools into one package, with modifications to make it build on Linux. It therefore allows reading/writing XMP metadata for various file formats, including PDF, MP3, JPEG, and PNG.
Exiftool is a perl-based module for embedding and extracting Exif from multiple filetypes. This allows for using XMP in addition to other formats.
XMP Manager A Ruby action for Nautilus which exposes and allows editing of XMP metadata.
Raptor - Supposedly this can generate XMP somewhat, and can also parse it, if you grab the rdf/xml out of the <?xmp...> blocks for it. It's GPL / LGPL / Apache2 licensed so compatible with your code.
XMP PHP Toolkit Extension - is a PHP5 extension (written in C++) including the Adobe XMP SDK. The main classes and functions from the Adobe XMP Toolkit are now availaible through PHP5. A 2.0 stable release is available under GPLv3 License, compatible with FreeBSD, Debian & Mac OSX
Python XMP Toolkit - is a Python library allowing access to reading and writing of XMP in a variety of file formats.