Haiku is an attempt to re-create BeOS in a modern context.
BeOS was a "new" operating system that came out after Mac OS, Windows, Linux, and the rest of the crowd. It was built from the ground-up on lessons learned from the shortcomings of the other operating systems, and was quite the promising OS. Unfortunately, it foolishly required Apple's PowerPC hardware... which was expensive.
But the biggest problem came when Apple refused to give BeOS the documentation needed to get it to work on their newer PPC machines (G3, I believe) and that was the beginning of the end.
BeOS was ported to x86 (standard, white-box hardware), but it was too late and the damage was done... So Haiku is trying to re-create BeOS from scratch in an open-source context.
BeOS and SkyOS are very similar in that they're both their own code, not *nix-derived, have very little market share, nice underlying architecture, are posix-compliant, and at the end of the day, are doomed to fail because they're not one of the big three OSes....