Other than ESPN360 which is fantastic, but usually a couple games behind, the best place I've found is:
Click on tennis, then click on "video" next to the match you want to watch, and then there will be a list of choices. Below the betting sites there are usually a few other links, some to JustinTV, some to UStream and some to VShare, and they all work just by clicking them. Those appear to actually be in time with the TV broadcast, not behind like the ESPN360 stream is. But the picture isn't as good.