Horde - Horde Groupware is a free, enterprise ready, browser based collaboration suite.
Freshservice - Freshservice: the one-stop cloud solution for all your IT management needs.
Cypht - All your E-mail, from all your accounts, in one place. Cypht is like a news reader, but for E-mail.
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
Mailpile - Mailpile is a modern, fast web-mail client with user-friendly encryption and privacy features.
Zendesk - Zendesk is a beautiful, lightweight help-desk solution.