Sonic Slam from Mattel is a pair of electronic stringless rackets with which you and a friend can play pretend tennis. You can also play tennis solo against a wall, or a game of hot potato, where the virtual ball explodes after a certain number of vollies.
What the heck, you ask? It works like a Wii controller without the Wii. The rackets detect how far you are from each other or the wall, and when you swing at the correct time the racket makes a realistic sound like connecting with a tennis ball. Or you can set it to make various loopy sound effects. They rackets have lights so that you can play in the dark.
Unfortunately they come with the annoying repetitive overly-excited narrator effect (“Your Turn!” “One point!” “Let’s Play!”).