Here you have a list of different home-cockpit forums. Please add to this list!
http://forums.avsim.net/dcboard.php?az=show_topics&forum=142 One of the most active international homebuilder forums
http://www.wcm.at/forum/forumdisplay.php?s=950ae9b63ef32878267255be20cec69d&forumid=71 Active German Homebuilders forum
http://www.simhardware.co.nz/forums/viewforum.php?f=8 The folks from down-under have their own
http://www.simviation.com/cgi-bin/yabb/YaBB.cgi?board=homebuild Mostly old plane stuff like Messerschmidt
http://www.hovercontrol.com/cgi-bin/ib3/ikonboard.cgi?act=SF;f=43 Something for the Helo folks
http://forums.frugalsworld.com/vbb/forumdisplay.php?f=48 One of the places the military/fighter sim folks meet
http://www.flightdeckbuilder.nl Dutch forum
http://www.fsnordic.net/discussion/index.php?board=33 Finnish forum
http://viperpit.org/forum/ F-16 specific cockpit builders forum