I hope this one makes it to its full potential. A heck of a lot more sites should have chat embedded on them than currently do. HN is a perfect example. We should have a "Chat" link in the navigation bar where we can hang out. We should be automatically signed in with our existing usernames.
A while ago I looked into doing this and were no good options. Mibbit was the closest but it's a bit ugly, doesn't have much of an API, and relies on IRC (which has pros and cons).
Personally I hate the Meebo-style bar they've implemented, but the actual chat is good and you can embed it in a page and disable the bar entirely.