openMosix is a a set of extensions to the standard Linux kernel allowing you to build a cluster of out of off-the-shelf PC hardware. openMosix scales perfectly up to thousands of nodes. You do not need to modify your applications to benefit from your cluster (unlike PVM, MPI, Linda, etc.). Processes in openMosix migrate transparently between nodes and the cluster will always auto-balance.
ImSafe (Immune Security For your Enterprise) is a host-based intrusion detection tool. After a learning phase, it is able to detect changes in processes behavior, to detect buffer overflows, etc. It is implemented through a device driver (as a kernel patch) for the Linux kernel, but can also be run on other UNIX systems by using a "sensor" built upon strace.