NDPPD
ndppd, or NDP Proxy Daemon, is a daemon that proxies neighbor discovery messages. It listens for neighbor solicitations on a specified interface and responds with neighbor advertisements - as described in RFC 4861 (section 7.2).
Current status
Version 0.x is in maintenance, and is being replaced by 1.0-devel which you can find here. 1.0 is not yet stable enough to be used in production, and I currently have no estimate when it is. Feel free to try it out if you like.
Latest stable release is 0.2.5: