Unknown Worlds Entertainment, the developers behind Natural Selection 2, have released part of a game they're working on called Subnautica. I write "part" because it isn't actually a playable game yet, rather, it's an awesome procedural generator of underwater worlds.

The big difference in how the Subnautica system generates worlds is that it does it on the fly. Most procedurally generated games (think Spelunky or The Binding of Isaac) require that the world be defined before the level is created. This is usually some form of randomised code or seed number that is randomly generated. The tool contained in the Subnautica Terrain Test allows you to load up your world, float around in it, and then edit the code without having to reboot. Instead, it will reimagine the world as you watch.

Unknown Worlds hope that releasing the tool will give them an idea of how the system performs on different computer builds and gain them some feedback on what they need to address as they continue to develop Subnautica.
The Subnautica Terrain Test is available as a free download on Steam, and a guide to using the kit is available here. Don't forget your snorkel.