Infoblox just started offering the IPAM portion of their software for free,
http://www.infoblox.com/en/resources/software-downloads/ip-address-management-freeware.html We've been using the full-blown commercial appliances (IPAM, DHCP, and DNS), not the freeware. I don't know exactly how it works without the other pieces integrated, but it may be worth a look.
