Many projects (such as MythTV and XBMC) have depended IMDB for a long time to provide metadata. Unfortunately this has usually been against the rules and was often difficult any since it relies on rather fragile screen scraping.
So in steps the Movie DB. From what I can tell from reading around the forums and stuff (and apologies if any of this isn’t accurate) it was started by the guy who wrote Meligrove, a database of fan made movie posters to replace a similar effort called Movie XML. The content is a little limited right now (about 8000 movies apparently) but the database from Movie XML is soon to be imported adding another 150,000.
Of course the important point is that the Move DB is an open database – all the entries are editable wiki style. It also has high level support for TV series (with seasons and episodes etc) as well and a not-quite-ready public API.