I can't say I know any specific gels or conditioners that encourage shrinkage, but I do know a couple of techniques that cause my hair to shrink more.
First, applying products to my hair when it is not soaking wet. If I apply products to soaking hair, my curls elongate, but if I apply products to just damp hair, it shrinks up. I believe this is because the products are not sealing as much water into the hair, so the hair is lighter.
Second, plopping. The longer I plop, the more shrinkage I get. I'm assuming this is because the having the hair piled on top of the head means that its weight is not pulling the curls down.
I agree with CurlieCutie that protein can increase shrinkage for some people. This is a great idea for a thread and I hope others chime in with condish/products recs!