There are over 100 different chat products so I don't think you can really distill it down to any single one that's used by the majority. Take your pick: http://www.capterra.com/live-chat-software/
They are on that same page but you have to weed through them all to find the most appropriate ones. Some common ones are: http://www.123flashchat.com/ http://www.hipchat.com/ http://www.adobe.com/products/adobeconnect.html It really boils down to what your site is built with, and if you need to integrate the chat with a certain membership software and payment gateway. So for example, if your backend was wordpress, it might be more wise to install one of the wordpress chat plugins so the room will integrate well into the site and member system instead of buying a standalone product that you'd need to write custom code to integrate.