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.
Roundcube - Web-based IMAP email client
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
Mailo - Mailo is an email client where you can send and receive emails to and from anyone with an email address.
Zendesk - Zendesk is a beautiful, lightweight help-desk solution.