Check out this article on LWN.net.  The author, Jonathan Corbet, describes how Google is customizing the Linux Kernel.  One of the most interesting facts was “Google runs something like 5000 threads on systems with 16-32 cores. ”  Pretty incredible when you see the big issues they have to over come to give us all are searches in less than a few seconds.  A neat article to remind most of us just how much we could be doing with the kernel if we really wanted to.