OpenCPN Radar Overlay PlugIn
Version 1.2 of the Radar Overlay plugin for Garmin radars with an Ethernet connection, has just been released. It's available from the plugins download page http://opencpn.org/ocpn/downloadplugins.
Follow the discussion in this thread http://www.cruisersforum.com/forums/f134/opencpn-radar-overlay-plugin-79081.html. There are versions for some other brands of radars under development.