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.
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.
