I can crack the following knuckles:

thumbs (both sets of knuckles) - on demand
fingers (both sets of knuckles) - pretty much on demand
ankles - on demand
knees - on demand
elbows - occasionally, if I've been sitting still for a while
ribcage - very occasionally first thing in the morning

My favourite trick is to crack my fingers against my forehead and convince children that I am cracking my skull.

How do you feel about knuckle cracking? My gf has got used to it but still winces every now and again, usually when other people are around. I think it embarasses her. My colleagues have never mentioned it but I was once party to a conversation about annoying noises that various people in the office make. I'm sure there are some people seething inside.