The Domesday Disk was a huge project of the BBC in the mid 1980's, intended as a modern day version of the Domesday Book, a complete census of the "State of the Nation".
Instead of being in a book, however, all the content was stored on two LaserVision 12" double-sided video disks. The entire package contained a BBC Master computer, the Video disk player and the disks themselves, and retailed for over £5000. For this reason, very few of them were ever sold, and the few that were, were mainly to fancy schools that could afford it.
The BBC micro wasn't capable of displaying the high resolution moving pictures, so the connection was quite complicated. The computer connected to the video disk player both with a data link (to control the player), and also the video output. The video disk player then played the images (still or moving) off the disk, overlaid the computer output on top, and sent the whole thing to its monitor.
So what was on it?
In summary, a hell of a lot. One newspaper recently reported "Thousands of schoolchildren helped record festivals, events and details of ordinary life, which were stored on 12-inch laser discs. They contained more than 250,000 place names, 25,000 maps, 50,000 pictures, 3,000 data sets, 60 minutes of moving pictures, and an unknown number of words. Around a million people contributed."
From personal experience, it had maps of the entire country, pictures taken of people, places and things, and pages and pages of information. A lot of the stuff was also short videos of people describing and event or a place.
It was certainly very cool to have at school, although I'm not sure how much "official" use it actually had.
A recent issue, however, was revealed. Despite the original Domesday Book still being readable, over 900 years since it was written, the Domesday Disk isn't any more. This is because the BBC computers themselves are long obsolete, and the few disks produced are becoming damaged. There has therefore been a project to collect up as many disks as possible, and attempt to retrieve all the information off them and re-render it into a more modern format.
Interestingly, people often cite this as a problem with digital data - it actually degrades faster than written data. I would argue against this. The degrading isn't because it's digital, it's because they picked a format (the BBC computer) that was proprietary and became obsolete. Programs written for the IBM PC in the late 1970's run on today's Pentium 4 machines, because the standard has been maintained. That should happen with all newer systems too.