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.
Zendesk - Zendesk is a beautiful, lightweight help-desk solution.
Telegram - Telegram is a messaging app with a focus on speed and security. It’s superfast, simple and free.
Freshdesk - Freshdesk is a cloud-based customer support software that lets you support customers through traditional channels like phone and email, social channels like Facebook and Twitter, and your own branded community
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.
HelpScout - Help Scout is a simple, straightforward way to provide excellent support