I can crack the very bottom of the thumb/side of my hand by holding it all the way around with my other hand and forcing the thumb upward. Usually the other thumb is placed right in or under the joint to get it to pop in.
Good news, it's been a few days of constant thumb cracking and the pain seems to have gone away. I suppose it was because I really upped the frequency of cracking (from only a few times a day to nearly every minute). On another note though, it tends to feel a little sore in the morning, but feels much better after a hot shower. Maybe warm/hot water helps with joint pains?