Searching for free live chat service

Many of my clients asks for integrating a live chat to their websites and mobile apps. I think coding it from scratch will be a lot of work so I tell them that I can't do it.

Is there an integrated live chat service which is free, even for a limited number of users?

Of course it would be helpful if it is easly integrated with variouse types of websites and mobile apps.

Any suggested links will be welcomed.