This eBook on Arduino Robotics is really worth your time to look over as it deals with all sorts of fun projects like a wall following robot, segway type robots (list below) and just so much more. If you like it I hope you track down a real copy that you can hold in your hands.
Paperback is just $13.95
Click here to sample this book.
Linus the Line-Bot
Wally the Wall-Bot