I must've missed this thread somehow over the years, but I think most of mine qualify, having started life as regular bicycles, and then meeting me and getting chopped up to build my creations from:
CrazyBike2 started out as a little BMX frame up front, and an old Schwinn 10speed in back. Early on it was pretty obviously two bikes, but later on it's less obvious as more of those have been replaced with more custom built frame bits for stiffness:
http://www.endless-sphere.com/forums/viewtopic.php?f=2&t=12500
early version
more recent, nearly how it is now (ignore the dog food cargo strapped on it in various places)
with a trailer full of stuff
SB Cruiser has little of it's original bicycles; the headtube, toptube, and downtube of some old Murray are there, albeit hidden under / inside other frame bits, and nowadays there's almost zero of the BMX frame that used to be under the seat, back when this used to be Delta Tripper:
http://www.endless-sphere.com/forums/viewtopic.php?f=2&t=67833
Delta Tripper was mostly normal bicycle; BMX frame and fork plus a Schwinn trike kit rear axle, some noname bike's seattube and BB stuck out front on a seatpost welded to the frame, and the seattube off some other 10-speed as a steerer tube:
http://endless-sphere.com/forums/viewtopic.php?f=2&t=22720
Unfinished cargo bike that began as a trek frame and a BMX frame and some other bits; dunno if I'll ever do anything further with it now that I have SB Cruiser:
http://www.endless-sphere.com/forums/viewtopic.php?f=28&t=31255