Here is Software Update Controller (xhost7) for machines on my LAN. It won’t work outside my local network, so don’t bother trying.
Update March 2017: Now supports through OS X 10.12 (Sierra) and probably beyond, as Apple seems to have standardized the catalog name.
Update November 2019: I’ve turned the Software Update server off, since I don’t need fast downloads when working on customer machines anymore. The script is still here, if anybody wants to learn anything from it.