I just got OS X server the other week and it comes built in with that, it sounds like you are looking for a cheaper solution though. I would search google possibly for dhcp freebsd or openbsd, you may be able to port over some open source solution pretty(relatively) easy. If you have success or find an app please share your success, I would love to do it at home, just been screwing around with Server so much haven't had anytime to work on my home machine .
If you have MacOS X Server - you configure DHCP through the Server Admin tool and NetInfo Manager.
This will cover off what _most_ people want to do. There are a few things that are more sophisticated that you need to tweak on the command line at the moment.
If you want to run a DHCP server on the non-server build of MacOS X , then I'd expect most off the shelf FreeBSD stuff to compile at worst with a bit of tweaking. I'm not sure if its been done already or not. This will likely be an all command line configuration.