MKVTagger
An unfinished and currently non-functional tool to tag MKV files (currently only movies and tv shows planned). Scrapes sites like TVDB and Themoviedb for information about the media, might support manual editing later as GUI is implemented and matures.
An existing tool exists with similar functionality but without the planned ability to download and attach cover art.
Requires MKVToolnix (mkvextract and mkvpropedit), as well as api keys for some scraping functionality (might be supplied in possible binary releases, not in source code)