Is there free open-source Live Chat software

I'm a freelance web designer who needs to communicate with remote clients. And instead of phone costs or having the clients signup and install instant messengers, I figure it would be great to have a "click to chat" button on my site where I can chat with clients in real time. Of course, we have to schedule an agree time before hand via email as I will not be online all the time.

Is there such software that is free that is similar to say LivePerson or LiveChat that I can install on my webhost.