What is this painful little bone jutting out of the inside of my wrist?

Hard to say: You can get a ganglion cyst that may form on the wrist and it can be painful. It is benign.

End of ulna: Likely the end of the ulna, the forearm bone that extends from the elbow down on the pinkie side, if it's the one i think it is (i spent 5 minutes poking, figuring out which bone you were talking about). If it seems different or if it's in a different place, you may want to point it out during your next appointment.

Need information: This could be many things such as a tendonitis, chronic repetitive strain, a small fracture or a ganglion cyst show it to your doctor if he isn't sure he will send you to orthopedics or physical medicine.

Probably the Hamate: If the area in question is along the little finger side of your wrist at the junction with the 5th metacarpal then you probably have a painful hamate. The bone is subject to hairline fracture especially by golfers. The pain is made worse by gripping.