I have always used either LiveZilla or Comm100. Both are free. Comm100 now has an app for iPhone as well
I personally like liveadmin.net it has a number of good features and a variety of implementation options from a free stand alone version, to a hosted solution and a pay standalone version. The only negative side to the package is that it utilizes flash on the front-end.
I use phponlinechat. It has all the features such as multiple department and operators, advanced live chat settings, operator rating etc. I actually downloaded the free version and am using it right now on my website. It works good.
Free Version - Helpden helpden (DOTCOM) is a great free chat software which is online based and has premium version as well. Good Live Chat Designs for Site ! Rating *****
Live person is one of the best software to use and best for chatting. Its very popular among the users due to its lots of clients
According to me, LiveChat is powerful live chat support software that helps you talk to your website visitors and convert them into customers. Also, This application uses cloud technology to give you flexibility and mobility.