Using xattr on the CLI you can write arbitrary metadata to files. This is useful for other CLI scripts because you can search for these arbitrary attributes using mdls. However, if you want to set metadata that is easily searchable in applications like Finder, you have to set the values for existing attributes.
$ xattr -w "com.apple.metadata:kMDItemStarRating" "10" filename
http://hints.macworld.com/article.php?story=20110314131100623
tagChimp want to make you life easy. Me store all DVD meta tags for movies, TV shows and music videos. Use popular programs to lookup tags at tagChimp, then put tags in you movies. Muy easy!
MetaX is a meta-data tagger for MP4 files and their derivatives, as well as QuickTime movie files. It uses a modified version of Atomic Parsley at its core to write the tags.
http://www.kerstetter.net/index.php/projects/software/metax
AtomicParsley is a lightweight command line program for reading, parsing and setting metadata into MPEG-4 files supporting these styles of metadata…
http://atomicparsley.sourceforge.net/
Not satisfied with the existing GUI front ends for AtomicParsley I wrote my own. It’s slimmer than the others I used by splitting the movie types into separate displays to offer a cleaner and smaller UI. Additionally instead of offering everything and the kitchen sink I opted to offer a smaller subset of the available options but ones which I feel are the core fields for the most common video types.