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Facebook. The whole experience is so tight.

And the chat program/shortcut bar at the bottom is very impressive, and must have been so hard to get right cross-browser.

I agree with this. I think Facebook's design is great because it is:

* Consistent across the whole site (for the most part)

* Clean, there are no gradients or extra flashes

* Fast, they focus on things loading quickly and make the site feel as snappy as possible