Most of them use livezilla, but they are too costly nowadays. Give a try this php chat software www.phponlinechat.com
.. They got both free and licensed versions. Moreover rates are quite affordable.
Single license - $50
2 license - $99
3 license - $140
5 license - $200
Unlimited license - $400
An ideal product if you want to use the chat tool for a single website. This product can be used to support customers online in a single website.
An economy product to use the chat software for two websites. This single product can be used for 2 websites.
A money-saver pack for businesses with three websites. This is a very ideal product to be used if you want to support customers in 3 different websites.
A jumbo product which can be used for as many as five websites. This PHP Live support software is perfect for enterprises which handle up to five websites.
An ultimate product to provide online chat support for any number of websites. This one-time purchase can be used for 'n' number of websites.