Horde - Horde Groupware is a free, enterprise ready, browser based collaboration suite.
Absence - Employee vacation tracking system
Roundcube - Web-based IMAP email client
CWS TimeOut - CWS TimeOut is an online leave tracking software or absence management solution that can also be installed on the server.
Mailo - Mailo is an email client where you can send and receive emails to and from anyone with an email address.
PTO Ninja - Slack app helping teams better manage time off