Of all defunct OSes, BeOS may have had the most promise. It emphasized simplicity, elegance, and support for parallel processing. The cancellation of development in 2001 upset BeOS fans.
Enter Haiku, a free, open-source BeOS. Over the past 10 years, Haiku developers have made impressive strides, despite the fact that the OS is still in an early alpha state. Due to Haiku's BeOS compatibility, many quality applications are available for this actively-developed OS. If Haiku continues to improve, the OS could become a mainstream, alternative desktop OS on par with Ubuntu Linux.