ClientBase - Travel Agency
SOGo - SOGo is groupware server with a focus on scalability and open standards.
Unit4 for Travel - Gain efficiency, live change and increase profitability with end-to-end ERP for travel management companies. Learn more about Unit4 for Travel.
Baïkal - Cal and CardDAV server
Travefy - The easiest online and mobile group travel planner!
Xandikos - Lightweight CalDAV/CardDAV server on top of Git.