Exiv2

This project is the best library tool I have found for handling image metadata.

Very complete API, and the support is much better than even commercial applications (this is for you, Robin Mills). However, this project is free and open-source.

The only weaknesses I see are that it's written in C++, which makes for some technical and design constraints, and its GPLv2 license is somewhat restrictive.

Apart from these, here is from one happy user.

For my favorite raw managing and editing software darktable, exiv2 is a very important library. While exiv2 does its work mainly unnoticed under the hood, it is a corner stone in the process. Additionally once in while I fire up exiv2 on the command line to examine some meta data in a photo.
The program works flawless and providing sample data gaps in the recognition of meta data are happily closed by the developers.
I'd like to thank all the devs for their work on this often not seen but very important piece of software.

I have been working with exiv2 lately and their support is awesome. Robin Mills gave lot of his time to fix some bugs with mingw version. He even made time for a personal chat session to get things working.

darktable uses this tool in the background, but it is also an extremely versatile command line tool for batch renaming and time adjusting your photo collection. It can do much more but I use it mostly for this.