Here's a link to Ben Ellison's weblog on marine electronics. Ben is a freelance sailing writer in Maine. He is constantly testing, and writing about, state of the art marine electronic gear.
http://www.panbo.com/
He is currently researching "AIS radar" which is a promising new transponder technology for maritime use. It's based on a vhf transmitter that broadcasts NMEA data from a vessel's GPS to other vessels equipped with a receiver.
Si-tex is currently selling the equipment.
http://www.si-tex.com/html/ais_radar.html