CometChat is one of the leading chat solutions to enable your site/app users to chat. We support cross platform chat across web, mobile & desktop. So your users can be on your website, or in your app and they can all chat with each other. We also have a Desktop Messenger (for Win & Mac) which will enable your users to chat directly from the desktop. So effectively, your users can login with the same credentials as your site/app and they can then chat from anywhere.
Here are some links that might be of interest:
10% off CometChat purchaseJoin to get access to this deal and 150 more!