I just came back from the Christchurch SuperShed (our city council recycling centre) where I bought a discarded slide projector for $6. In less than an hour, I turned the projector lens into a tilt macro lens for my dSLR. I know there have been lots of these sorts of projects but I tried to do this one without hot glue or gaffer tape so hopefully my process will help you to turn your odd lenses into useable ones.
Step 1: Finding the right lens
Any lens can be adapted as long as the distance from the back of the lens to the focal point (slide, film, or whatever it is projecting from or to...) is more than the distance from the flange (where the lens attaches) to the sensor. Don't stick a ruler into your camera, just look it up. For my camera (Olympus 4/3rds), the flange distance is 38.67mm (Wikipedia - Lens Mount) and the back of the slide projector lens to where the slide would be is around 45mm.
This lens has a focal length of 85mm, and compared with some of the other lenses I saw, I guess it is about f2.8. I should point out that most projection lenses do not have variable apertures so you have to shoot with them wide open (shallow depth of field). This one has a spiral groove which makes it easier when enabling twist focus.
Step 2: Additional Materials
In addition to the lens, you'll need a length of hose and a body cap for your dSLR.
Step 3: Determining Infinity Focus
Hold up the lens to your camera and point to something in the distance. Move the lens in and out until the distant objects come into focus. This lens came into focus around 10mm out from the camera flange. Measure the distance from just inside the camera flange to the end of the lens and that's the length you need to cut your hose.