I really like Haiku, they didn't just blindly recreate Beos, they gave it a few modern feature as well. It's hardware requirements are low, it's organisation logical and yet it's not completely foreign to users of modern OSs. I know the concentration right now is on developing a desktop version but I hope they consider making a version for mobile devices.
In a way it's a lot like what I would have expected from Amiga OS had it's owners survived the '90s and continued to develop the OS.