If your looking for a free Live Chat product then I also suggest LiveZilla.
Zopim - We have used Zopim, but based on customer feedback switched to another solution; we found that customers thought the chat bar was frustratingly small. We did contact Zopim regarding this but they said it was not possible to change the size.
ProvideSupport, LLC. - Our current solution, we are finding it brilliant. The operator console works with Windows, Linux and MacOS, chat window can be customised, responses can be customised, they have great support, and based on customer feedback its perfect
Kayako - We have tried Kayako, however found the new pricing structure is just way to expensive and outweighs the benefits; we find that WHMCS/ProvideSupport work perfectly for Helpdesk and Live Chat.