Haiku is another open source operating system, and IMO we can say it continues from where BeOS left off. I haven’t had a chance to try BeOS, but read about what it was supposed to do and some beautiful screenshots. BeOS was a closed source OS, and some loyal users tried to re-create the OS under OpenSource license.
And there came Haiku OS, an open source OS, and it released its alpha version on 09/2009. It is written in C++. The ISO image, as well as qemu/vmware images are now available to download. I just did a test run via their live CD image, and I would say I’m quite impressed. I wish it’d continue to R2 release soon.