With OpenMosix development now halted (and still stuck on 2.4 kernels), there are two similar projects that allow a cluster of computers to appear like an SMP system (eg. processes are distributed across machines, global cluster PID, etc).
Kerrighed (Patches the 2.6.20 kernel)
OpenSSI (version 1.9 patches the 2.6.11 kernel).
Feature wise, Kerrighed has the upper hand. According to this features matrix, Kerrighed can migrate threads across nodes, while openmosix and OpenSSI cannot.

