For an entire generation, Richard Donner's Superman defined what a comic-book movie was and Christopher Reeve's portrayal has been the defining iteration of the character.
The film was such a success that, in 1982, ABC put together a three-hour extended cut of the film which featured never-before-seen footage and more cues from John Williams' iconic soundtrack. The film played out over two nights, and became something of a legend for Superman fans. A certain amount of the footage made its way into the various home releases, either through deleted scenes or the like, but now it's confirmed that the full, three-hour version of the film will finally get a Blu-Ray release pretty soon.
Superman: The Extended Cut features the full 188-minute version that was shown in 1982, with a brand new 1080P HD mastering of the footage as well. On top of that, there's also a copy of the Superman: Special Edition which features commentaries (remember those?) from Richard Donner and Tom Mankiewicz, a music-only version of the movie, and tons of featurettes and documentaries in there as well.
If you're a Superman completionist, this is one to pick up.