Zulip - Chat for distributed teams. Zulip combines the immediacy of real-time chat with an email threading model. With Zulip, you can catch up on important conversations while ignoring irrelevant ones.
GitHub Team Discussions - Now there's more space to talk through your projects 💬
Flow - Stop managing projects from your inbox.
Schnack - Schnack is an open source commenting system written in JavaScript.
Chatfully - Chatfully is one place for every chat.
Product Hunt Reviews Widget - Embed and encourage PH reviews into your product