This is the best place to watch tennis on the interent. For a small subscription fee you get to see pretty much every tennis competion going. No Slams I'm afraid, but those are usually covered by normal TV.
This channel on TV and the Internet always has a good coverage of the Slams as well as some of the other tournaments. This year it had great coverage of the Australian Open, showing all the big matches live.You can access the TV channel for free with a Sky box. The internet player requires a small monthly subscription.