They're called ball joint seperators. See picture:
FWIW, I had a customer once with a Chevy van that needed a new steering box. I had to pull the tie rod off the ball joint on one side. I tried for an hour to get them apart with a pickle fork and a single jack sledge hammer with no luck. The owner came out and handed me a ball joint seperator that was his personal tool, and let me use it. Before it was even tight, the tie rod popped off. That afternoon I went and bought two of them.
Take care,
-Chris