Hide X is a command line file manager-ish software, released initially September 11 2001 (don't quote me on the date, but I'm pretty sure.), that creates a virtual storage area. It's under a custom open source license called the L.O.P. (Lax Open License), by the general community. The official website was hidex.oss (see OpenNic Project for more information about .oss domains), but isn't any longer. Still waiting for a new site to appear...somewhere...hopefully?
I don't totally understand how it works, but the basic idea is that you use specified existing files on your computer (or stored remotely) to create a library of code that Hide X interprets, breaks into pre-defined chunks, and assigns keys to. When you save a file to the hidden storage area it's interpreted, broken into chucks using the same interpretation in your library, and then a sequence of the required keys is written to the database kind of like a compressed archive.
I say kind of like, because unlike a normal archive the database entry doesn't contain any of the actual file data, it contains the data needed to re-create the file, the keys.
The idea behind Hide X (as I'm sure you can tell by the name) is to provide hidden secure storage. Unless someone is aware that Hide X is installed they cannot find your data. The database is encrypted, and as discussed it doesn't actually have any of the original code in it (so the database on it's own is useless), and Hide X requires a login before it can be used.
You can have multiple databases which can use the same or different libraries. Other software can be built to have support for Hide X's database. Several years ago a few people on the forums banded together, and made a fork of VLC that could playback files stored in the virtual storage (technically not, since the database is used to re-create the file and then the file is stored in memory). There was discussion of building a custom linux distro with built in support so that applications wouldn't need support added to them individually, and so that users wouldn't need to use the cli to manage files.