Also, because it uses a standard chroot'able file system for every node, it is extremely configurable and lends itself to custom environments very easily.
Warewulf is more along the lines of a distributed Linux distribution, or more specifically a system for replicating and managing small, lightweight Linux systems from one master.
Using Warewulf, HPC packages such as LAM/MPI/MPICH, Sun Grid Engine, PVM, etc.
Because of this it is as flexible as a home grown cluster, but administratively scales very well.
As a result of this flexibility and ease of customization, Warewulf has been used not only on production HPC implementations, but also development systems like KASY0 (the first system to break the one hundred dollar per GFLOPS barrier), and non HPC systems such as web server cluster farms, intrusion detection clusters, and high-availability clusters.