I haven't watched tennis in a while, so I wasn't really familiar with the new Shot Tracker challenge system that has been implemented.
Basically cameras are set up in a 3D array track each and every ball. "In" and "Out" calls are still made by line judges. The players can challenge the calls the line judges make, at which point a 3D computer generated screen shows up on the jumbo-tron showing the exact path of the ball and making a mark exactly where the ball hits.
Players get two challenges per set and only lose one if they challenge a call that initially was correct. An additional challenge is added for a tiebreaker.
I think the system is fantastic. It adds a dramatic effect to the match. No more do we have to see player arguing with the chair umpire about calls. Most players seem to like it also. Roger Federer is one that does not like it. He claims that using the Shot Tracker system shifts the responsibility for overturning calls from the Chair Umpire to the player. There is a good writeup about the challenge system at ESPN.
What are some of the other great sports tech innovations? I think the 1st & Ten system in college and pro football is a huge advance. Yes, that's the name of the computer generated yellow first down marker. The image looks so good, it appears as though it's painted on the field. A lot of times it's easy to forget that the players don't have the advantage of the yellow line. The yellow first down line makes football imminently more watchable. The Wikipedia writeup on the 1st and Ten System.
Another cool technology was the FoxTrax hockey puck in which a circuit board was inserted into the middle of a puck. Fox was then able to track the puck and add cool laser trails to the puck on the broadcast to make it easier to track. Many players said it changed the feel of the puck, however. Wikipedia writeup on the FoxTrax system.