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http://www.filebot.net

Very powerful file renaming tool that also leverages TheTVDB, AniDB, TheMovieDB, etc for TV and movies. Both GUI and command-line versions available, I always use the (free) command-line version even though I paid for the GUI.

It also accepts scripts, has smart/fuzzy matching, and is an all-around good renaming tool for other things too.

For television, you might find http://thetvdb.com/ does what you want.

If your filenames are already decent and you just want to be able to better browse/manage your content, I fully recommend Plex. It'll pull down names, artwork, descriptions, and everything else.

tvnamer uses thetvdb.com's API in a similar way to picard / musicbrainz.

It's a great tool. I had one problem / wishlist that I couldn't work out, and got a response from the author a few minutes after filing an issue, explaining how to solve my problem.

There's some risk that thetvdb.com's change of API later this year may be a problem, but I suspect a low risk.

https://github.com/dbr/tvnamer , with packages available for most distros AFAIK.