Absence - Employee vacation tracking system
Trello - Infinitely flexible. Incredibly easy to use. Great mobile apps. It's free. Trello keeps track of everything, from the big picture to the minute details.
Appogee Leave - PARIM Workforce Management Software
Nifty - Manage projects, work, and communications in one place.
CWS TimeOut - CWS TimeOut is an online leave tracking software or absence management solution that can also be installed on the server.
CAMPR - Open source project management tool with 16 modules