WhatsApp - WhatsApp Messenger: More than 1 billion people in over 180 countries use WhatsApp to stay in touch with friends and family, anytime and anywhere.
Question2Answer - A Q&A site helps an online community to share knowledge.
Telegram - Telegram is a messaging app with a focus on speed and security. It’s superfast, simple and free.
The Answer Bank - The Answer Bank is the UK’s leading question-and-answer site.
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.
Stack Overflow - Community-based Q&A part of the Stack Exchange platform.