Intercom - Intercom is a customer relationship management and messaging tool for web businesses. Build relationships with users to create loyal customers.
Talkus - Chat with your website users from Slack
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
Speaklyn - Website visitors chat with you on Facebook Messenger
JivoChat - JivoChat is a live chat that enables you to talk to visitors on your website
WhatsHelp - Communicate personally with your customers via Messenger