HelpScout - Help Scout is a simple, straightforward way to provide excellent support
UseResponse - Open-source, self-hosted customer feedback software, live chat and helpdesk system that you can install on your server. Organize documentation using knowledge base and get feedback from social networks with centralized system
Otr.to - A secure peer-to-peer chat in your web browser.
tawk.to - tawk.to is a free live chat app that lets you monitor and chat with visitors on your website or from a free customizable page
Skype - Stay in touch with your family and friends for free on Skype. Download Skype today to chat and call on desktop and mobile.
ChatWoot - Open-source omnichannel customer engagement platform