AD:DNS
AD:DNS is a simple DNS proxy application designed to selectively proxy DNS requests to different DNS servers based upon the IP address of the requesting machine.
It was developed for internal use at Appcon where we needed a way to have selective filtering for our users. We needed the top brass to be able to view basically anything, while still locking down the other users.
We also had no budget. 
You can find the bits for AD:DNS on bitbucket here: http://bitbucket.org/wedtm/ad-dns/