I've been using Talkinator, an embeddable chat program for websites, for a few months now.
I realize I might be the only post-1995 site to want a chatroom, but it's actually rather useful. For example, when people are discussing, say, Microsoft Interview Questions, they'll jump in the chatroom to discuss problems. This use was expected.
The more interesting use-case was simply feedback. People hesitate feedback via email, or even through anonymous forms. They will, however, jump in a chatroom and complain. I've discovered a number of bugs this way.