Command line media
Open source command line media tools -- this page is by no means complete.
Frameworks / Libraries
Frameworks (and libraries) are generally speaking functionality that is made to be used from code (so like plugins to a programming language). Many frameworks also come with command-line tools (often for testing the framework) which can be quite useful even without getting too deep into coding.
- exiftool Reading and writing file metadata (and more than just the "exif" standard)