As of earlier this year, ZETA (YellowTab) is as dead as BeOS itself. They didn't actually have permission to be using the BeOS source code (surprise surprise) and hence had to stop distributing it. The BeOS legacy isn't completely over though, as Haiku still seems to be going:

http://haiku-os.org/

It's a complete, ground up, rewrite that aims to be binary compatible with BeOS...