Prodigy
Prodigy was an online service mostly active in the early 1990s, formed originally by IBM and Sears as a consumer videotex service. The transmissions (with text and graphics) were primarily based on NAPLPS, but had many proprietary elements, and required special Prodigy software to use. There were a number of active communities within the forums, including fan communities for various things (including teen pop crazes such as New Kids On The Block), but as the '90s wore on AOL got more popular and Prodigy faded out. In 1995 Prodigy was among the first consumer online services to make Web access available, using its own browser embedded in the Prodigy software, but ultimately the Internet eclipsed the service and Prodigy moved to a new "Prodigy Internet" service, keeping the old service around for a while as "Prodigy Classic" before finally killing it altogether in 1999.
STAGE.DAT
A notable file-format-related issue was the service's storage of a file called "STAGE.DAT" on the user's hard disk (usually in a C:\PRODIGY\ directory, or C:\PRODIGYW\ for the Windows version), which cached various things from the service to improve access speed. This file proved controversial as it sometimes seemed to contain personal data from elsewhere in users' hard disks, leading to fears that Prodigy was spying on its users, but it was apparently just a side effect of the disk block allocation routines which sometimes led to parts of deleted files being present until overwritten by other data, and this data was not transmitted to Prodigy. More recently, that file got back in the news after it was discovered that parts of the long-dead Prodigy content could be recovered from users' STAGE.DAT files, like flies preserved in amber, making the files of interest to historians and archivists.
Researcher Jim Carpenter analyzed the file in 2014, determining that the file contained an entire filesystem of its own, which he discovered was in FAT12 format. Other details of what was stored in the file could be discerned by reading Prodigy's patent on the associated technology, US Patent 5,347,632. For a change, this actually works the way the patent system was originally intended; the patent monopoly is granted in exchange for the inventor giving full disclosure of how it works (not keeping it a trade secret) so that others can build on it and will be able to have unrestricted use of the techniques once the patent expires. Unfortunately, this is often defeated in modern patents by the use of inscrutable legalese making it hard to find any coherent technical facts in the patent text. In this case, however, Prodigy's techniques are outlined in very specific detail.
