[lutusp]

> What are the reasons people decide to start a new OS development project?

Dissatisfaction with existing operating systems? A desire to advance the state of the art?

> Why to maintain your own open source OS, and not to adapt or contribute to an existing OS?

You might as well have asked why people have children. Aren't existing people satisfactory? Why try something new?

> Who is behind these non-mainstream open-source OS?

What does "non-mainstream" mean? There is no main stream, either here or in nature. Evolution doesn't work that way. And OS development follows the rules of natural selection -- the fittest survive.

Look at Microsoft Windows -- it's about to be extinguished by its excessive size and inability to adapt to change. Just as with the dinosaurs -- because of their size, they were extinguished by an environmental change they couldn't adapt to. Smaller, more nimble creatures survived and took over.