Wimbledon has nothing on Gamigo's Smash Online.
Can't get into the mens or womens draw at the All England Club? If you are in Europe, try the fresh-out-of-beta English version of Smash Online from Gamigo. One of only a few in the F2P MMO sports genre that has launched, Smash Online provides the opportunity for character development in a sporting arena which seems like a natural MMO fit. Playing it safe though, Gamigo may be afraid of the Williams sisters pwning online like they do in real life, so the game is not officially available in North America just yet. If you are in North America don't despair, Smash Online has a short virtual fence you can jump over and still play, but you may have to restart your character when it officially launches in the region.
Garnering over 50,000 sign ups to date, Smash Online:
"Smash Online is an easy to pick up sports game with elements of Online Roleplaying Games and attractive 3D graphics. Play matches against human opponents from throughout Europe, develop your character skills and participate in international tournaments. Smash Online is the ideal game both for the lunch brake and long evenings, offering something to casual gamers and to sports game experts - and best of all, it is completely free to play. Players can register on http://tennis.gamigo.com and play within minutes!"
Being a former tennis player and current MMO player, both aspects of the game are very appealing. Once upon a time, Top Spin was the most played game in my house (Don't tell my raiding group though, please). For more information and one of the easiest account creation processes ever, go to Smash Online.