This was a fun undergrad term project that I did with a partner for a course on designing CAD systems. One of those great experiences where you forget about the assignment and just get consumed by the project itself.
RollerCAD was a 3-D design and simulation software for building roller coasters. Define a track with control points, specify which way is up, add loops and corkscrews with a click. and test the whole thing out by putting a cart on the tracks. See what it looks like from the view of a rider, or have the camera pan to follow the cart. See where the g-force will exceed what riders will enjoy (or survive), and tweak the design to make it better.
The software doesn't run anymore, but I did find a few old screenshots.