I am using LiveZilla ( an open source) and its free.
Thought you may be searching for such great script.
You can download it from here LiveZilla - Freeware Live Support / Freeware Live Help - Official Website
Now you can chat with your visitors and hopefully, you can convert them as a customer
If you want to see the demo then come on my site I will show you how it works. ( use pen name Warrior-test. ( url in my sig )