H.323 = IMTC/ETSI
SIP = SIP Bakeoff aka SIPit (pillsbury got upset)
MGCP = Intl softswitch consortium
These are the groups that hold annual or semi annual interop events. Whole ton of companies are on each board.
sip = http://www.sipforum.org
mgcp = http://www.softswitch.org
h323 = http://www.imtc.org