Hulu's high quality player will actually be streaming the event live. Downside? If you're not in the USA, you're going to be restricted from viewing.
CNN has a live stream on the site that could essentially be called CNN, the B-Side. There's a lot of interesting stuff that shows up on this stream in general, including a weekly feature by Chris Pirillo. Their debate coverage is generally the same as what's on the main cable channel, but the downside here is that it only works in Internet Explorer.
The coverage is pretty standard, and generally has that little "dial meter" reaction thing running across the bottom if you're into that. The upside about the MSNBC player is that if you've missed it, you can watch it in totality after the fact, and even embed it on your blog.
ABC's stream appears to be hosted over at Yahoo. The coverage on the ABC affiliate station in your area is likely the same as what it is here. There doesn't seem to be any pre or post-show, so if you hit the site before the debate begins, the player may fail to load.
By far, one of the better and more interesting options, they're integrating Twitter discussion, and are playing the stream with Mogulus's live platform.
Current is also attempting something along the lines of CSPAN. They're not only creating livestream coverage and including Twitter interaction, but putting certain tweets up on screen for both the online and satellite channel.