What are the available solutions for embedding chat functionality on a website?

We want to introduce chat on our site so that users can communicate with each other.

Our general requirements:

• It should display in our web pages, but it can display in an IFrame or something. Our site users are part of the general public, not internal teams, so we do not want them to install a separate application.

• Users should be able to use their existing account on our website and not create a new account for the chat tool. If we partner with someone like Meebo, we do not want to share a significant amount of user information with the partner.

• The code is publicly licensed, but preferably not an open source project using the GPL license, but a BSD or MIT license (and possibly others) is fine. An inexpensive product with a non-public license might be fine.

• We want this to be implemented pretty quickly, and we really don't want to create our own solution.

Has anyone worked or knew of any solution that satisfies some or all of these requirements? Any other ideas / suggestions?

Thank.