Twisted Tennis
Not all that twisted, really. You can score points both by getting the ball past your opponent and by belting something or someone in the background.
Yahoo Tennis
Nice low camera on this one. Good for judging ball height. And checking out your player's tush.
Crazy Tennis
In which Primitive Man attempts to assert his dominance over the animal kingdom by landing a 100-mile-an-hour return.
Da Bomb Pong
Loony-Tunes ping-pong with an explosive ball.
Galactic Tennis
This one's cheating - it's more table hockey than tennis.
Match Point Tennis
What can you serve but not eat? A tennis ball, silly. (Yes, we've run out of ideas.)
Smash Online
Forget five minutes of fun - this is a big, juicy massively multiplayer tennis sim. It's all free-to-play, but you might want to opt out if you dislike anime.
Super Chili-Timers Tennis
Not as good-looking as some of the games on this list, but among the most feature-heavy. Besides clay, grass and omni-courts, you can play on a space-station.
Tennis Breakout
Breakout, but with a tennis racket instead of a paddle. Legendary.
Tennis Game
They might as well have called it "Without Title". Thumbs down for creative wordcraft, but thumbs up for polish and simplicity.