I have a few BSD-based systems sitting around serving away, but for the most part they're idle CPU-wise. Does anyone knew of a bitcoin client that I could run on these machines to make them more productive while they wait for requests?
The standard Bitcoin client should run on BSD machines. If you encounter any difficulties getting it to compile or run, ask more specific questions.
there is a port available for freeBSD: http://www.freshports.org/net-p2p/bitcoin/