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.
Step 4: Cut a Hole in Your Body Cap
Cut, melt, and/or nibble a hole into the middle of of your body cap, slightly larger than the inside diameter of your hose. This will allow you to fix (or screw) your hose onto the body cap without glue or tape. Ideally, you should find a hose that the lens fits into but allows it to move and twist (to focus).
Step 5: Holey Body Cap
My hose was spiraled so I cut a notch on one side so I could thread the hose through it. If your hose isn't spiraled, you don't need the notch. Also, I'm using an old Konica AR body cap so I stole a hair tie from my daughter so it fits on a 4/3rds body. I use this same trick for all my Konica AR lenses but don't use the hair ties with metal crimps unless you like scratches on your lens and camera.
Step 6: Put It All Together
Screw the hose into the body cap and slide the lens into the hose.
If your tube is wonkey (it's not straight enough for your liking) try making a wider notch and/or sloping the edges of the notch to better fit the spiral.
When it all looks good, put the lens on the camera and point it at something in the distance and see whether it comes into focus with the lens pushed all the way in. If it's not in focus and moving the lens out doesn't fix it, your tube is too long. Cut off a few millimeters at a time and retry until until it's in focus.
Move the lens in for distant objects and out for close objects. For macro work, use a longer piece of hose and bend the hose around for the tilt function.
Set your dSLR to A or M and go take some photos....
Step 7: Adding Twist Focus
If your lens has spiral grooves (like mine), you can add a screw to the inside of the hose and now you can focus by twisting the lens.
If your lens doesn't have spiral grooves, and you used a spiral hose, you can add a screw to the outside of your lens (the part that is inside the hose) and it will also allow twist focusing.
Step 8: Sample Photos
Here are a few sample photos I've taken with the lens. Because it is wide open, it is difficult to focus near and distant objects at the same time, but that "Bokeh" is what some photographers are looking for. The first Altoids tin photo is to establish the scale, then I took the second photo as close as I could focus. This would be as "macro" as the original tube could be. As a test of "how close could I go" I replaced the original hose with a 300 mm (12 inch) section of hose and took the last photo. It was pretty unwieldy and I can't see many people walking around with a foot-long droopy lens on their camera but it was fun to try.