File system navigator, a 3D application for file management for IRIX. Released by SGI as freeware, and it's still available (as a binary-only release, though).
It displays files as columns of different heights, and lets the user to move to different viewpoints.
It's a rather cool program, while it's also somewhat useless. =)
Its main claim to fame is that it appeared in Jurassic Park. Many UNIX users attempted to laugh the character who said "This is UNIX! I know this!" out of the room, but the scene used a real UNIX program. Of course, one can always argue that the program doesn't represent UNIX user interfaces in general too truthfully...
If you have MovieOS believers around, you can show the Power of UNIX to them with a clone program called fsv.