The Collaborative International Dictionary of English v.0.48:

keel over \keel` o"ver\, v. i.
   To drop down in a faint, or as if dead; to die. [Colloq.]
   [PJC]

WordNet (r) 3.0 (2006):

keel over
    v 1: turn over and fall; "the man had a heart attack and keeled
         over"

Moby Thesaurus II by Grady Ward, 1.0:

40 Moby Thesaurus words for "keel over":
   be lost, black out, capsize, careen, culbuter, drop, faint,
   fall senseless, founder, go down, gray out, keel, overset,
   overthrow, overturn, pass out, pitch, pitchpole, plunge, scuttle,
   sink, slump, somersault, subvert, succumb, swoon, tip over, topple,
   topple over, topsy-turvify, topsy-turvy, tumble, turn a somersault,
   turn over, turn topsy-turvy, turn turtle, turn upside down, upset,
   upset the boat, upturn