I have a 1981 Fuji Royale that originally had a Suntour Cyclone rear dérailleur. The bike has a six cog freewheel and friction shifters on the downtube. The old dérailleur was starting to shift poorly, and the chain came off a couple times, so I decided to replace the dérailleur. I bought a Shimano Deore RD-M510 for $17 from Nashbar and it works great. It bolted right in place and I didn't have to replace any other parts. Shimano calls this a "mountain bike" part, but it is much better suited to an old road bike with large cogs than their new "road" dérailleurs. Apparently this model of the Deore dérailleur is a closeout item and they are available real cheap at several online vendors.
welcome. The Deore derailleur has been used in a bunch of non-mountain-bike type applications, including one in which we tried to build a variable rate system for lowering a camera into the Three Mile Island reactor core. We finally went electric, but I thought it was a really cool application, even if it didn't work.