Say you have a frame that doesn't have a rear dérailleur hanger. Or maybe yours broke off and you want an easy fix. A cheap/quick fix is to run one of these:
Note you might need to drill into your frame to secure it (ala the little hole in the part above, so it doesnt pivot)
It might not be elegant, but many bikes in the 80's came stocked with this part for derailleured bikes for some reason. Also, it would get you by in a pinch.
Some guy is selling them on Ebay, but I'm sure QBP or BTI or your local LBS could get you one too.
Anyone ever done this with success of failure? Reply in the comments.