At this month's GTALUG meeting, Scott Sullivan gave an introduction to Plan 9, developed as a research project at Bell Labs in the 1980s as a possible replacement for Unix. Plan 9 is mainly of interest to OS researchers and hobbyists. For the rest of us, the OS is quite limited. Someone asked if you could read your mail on Plan 9 but never got a clear answer as to how you might do it.
Linux incorporates a few concepts from Plan 9. Some, such as /proc, /dev, and /sys, are obvious, while others such as FUSE or unionfs, are less so. Linux directly supports Plan 9's 9p protocol, although no tools were mentioned that actually uses it. The Glendix project intends to run a Plan 9 user space on top of the Linux kernel.
Despite Scott's enthusiasm for Plan 9, in the end I was not inspired to look at it more closely. Plan 9 has some interesting concepts but none of it is very useful to me. Just being cool is not enough.