If you are interested in building your own cluster, here are some links that might be interesting for you. Of course you must know what cluster type you need (HPC, HA and so on).

http://www.bsdcluster.com/

http://redundancy.redundancy.org/fbsd_lb.html

http://turtle.ee.ncku.edu.tw/sgcluster/

http://ganglia.sourceforge.net/

http://oscar.openclustergroup.org/

http://www.lcic.org/software.html