The TTT robot is very easy to build and also one of the fewest robots that are actually useful, it can really serve as a trainer.
The TTT is made of 3 basic parts: the rotating base, the swinging arm and the ball magazine. You will need a Turntable for the base so that the robot can turn slightly from side to side.
I used one motor and connected it to the turntable through a standard Lego part (see pictures).

The main part of the TTT is the swinging arm that hits the ball. I used 2 motors for the swinging arm to give it more power.
The ball magazine can hold up to 9 balls at a time, you can make it higher to hold some more, but then I recommend that you make the base wider (it can get pretty rough while at work).

The program for this model is simple. The side-to-side action is random and two touch sensors are used to control it (I used home made sensors, but Lego sensors can do just fine). The swinging arm rotates for a brief moment, hits the ball, stop with a break and than float to let it go down again (with the help of gravitation).

Download the program for this robot (3 KB).
Download the building instructions (the following set of pictures) and a video of the robot in action (319KB).