ArduPilot (original version)
Specifically needed for a Sailboat - a sailboat only absolutely needs a wind vane the rest is optional
Other discussion of ArduPilot-based boats:
Lake Depth Survey Blog - http://diydrones.com/profiles/blogs/ardupilot-goes-into-the-water
Lots of interesting posts about the evolution of a boat used the survey the depth of lakes. He used the Ardupilot and V1.0 software, but modified it for a his boat.
Discussion regarding sailboat application - http://diydrones.com/forum/topics/imu-3000Didn't entirely understand the conversation, but it regards drift compensation.
Required components for a Boat - http://diydrones.com/forum/topics/ardupilot-boat-requirementsUser Jim Covert asks about the components need for a boat project. Chris Anderson points out the that he'll want to use Ardupilot 1.0 which lacks stabilization control. A boat doesn't really need that feature.
Another lake survey project - http://letsmakerobots.com/node/21098Detailed description of boat building process and application of an Arudino Duomilanove for controlling the boat.
Boat navigation with ArduPilot - http://diydrones.com/forum/topics/boat-navigation-with-adrupilotUser Tony K. asks about the components needed to pilot a sailboat. Chris Anderson Give some direction on what is specifically needed and some specifics about GPS parameters.
Two-way Telemetry - http://diydrones.com/forum/topics/moving-waypoints-in-flightUser Peter Ho appears to be using ArduPilot on a boat and wants to change waypoints while en-route. Chris Anderson points out that two-way is available on the ArduPilot Mega.
AP for Sailing Vessel - http://diydrones.com/forum/topics/ap-for-sailing-vessel
AUOOSB Open Ocean Sailboat Project - http://diydrones.com/profiles/blog/show?id=705844%3ABlogPost%3A6278...
Michael King Put up a community page at auoosb.ning.com to work toward building an autonomous sailboat. The site appears to have put on hold by NING but I believe I contacted Michael about reactivating it.