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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
Zammad - The Zammad - Help Desk and Ticket System - Community Page
ServiceNow - ServiceNow automates and manages global enterprise service relationships. Create a single system of record for enterprise services, automate manual tasks, consolidate legacy ITIL systems.
FreeScout - Free open source Zendesk & Help Scout alternative: help desk, shared mailbox, email management...