Phil Harvey’s exiftool [0]. Despite being a command-line tool, it is widely known and used by professional and advanced amateur photographers to extract or repair metadata in images. It does handle more than just still images.
It’s written in perl and the perl API for handling the metadata is documented, but I’ve never seen anyone use that directly.
I created an account here to say this: Exiftool doesn't only work with image files, either. I use it to extract metadata from audio files as part of a toolchain that creates playlists automatically from my music metadata.
It’s written in perl and the perl API for handling the metadata is documented, but I’ve never seen anyone use that directly.
[0] exiftool.org