Keep track of the films and TV show episodes you have seen. Follow shows and have your own TV calendar, and never forget your favourite TV show episodes.
IMDb - Internet Movie Database
Simkl - Simkl is a TV, anime, and movie tracker that keeps a history of all the shows and movies you watch in one, central location. It’s a mobile app, a website, Google Chrome extension to keep track of everything you watch and integrates with many TV apps
Letterboxd - Letterboxd is a social site for sharing your taste in film, now in public beta.
Trakt.tv - Automatically track TV shows & movies you're watching.
Rotten Tomatoes - Rotten Tomatoes, home of the Tomatometer, is the most trusted measurement of quality for Movies & TV. The definitive site for Reviews, Trailers, Showtimes, and Tickets
Criticker - The TCI is a measure of the similarity between two people's taste in films.