Horde - Horde Groupware is a free, enterprise ready, browser based collaboration suite.
ClearOS - A very good alternative for Windows Server, for free
Roundcube - Web-based IMAP email client
NethServer - NethServer is an operating system for Linux enthusiasts, designed for small offices and medium...
Mailo - Mailo is an email client where you can send and receive emails to and from anyone with an email address.
Univention Corporate Server - Univention Corporate Server UCS with Active Directory functions and App Center is the cost-efficient and popular successor to Microsoft's server solutions.