I've gotten many decent bikes from the local thrift shops:

1983 Mongoose Californian - got for $16, flipped for juuuuust a bit more than that


1997 Schwinn Hydraglide - $14, done over and given to a neighbor kid


1987 Mongoose Hilltopper - $20, i think, was my wife's, since given to one of her friends


1984 Centurion Elite RS - my current road bike even though it is too big for me


Other bikes I remember are a Tom Kellogg-built Ross Signature (gave to a neighbor) and a Nashbar Toure MT (also given to a neighbor).

I also recently grabbed a 1994 Mongoose Rockadile MTB for my son.

I also will buy cheap crappy big box store bikes at the thrift shop if the tires, chains, and pedals are in decent shape - they are a good, cheap source of small parts, and can usually be grabbed for less than the price of a new tire.