Its been mentioned in another thread but I've setup a sorceforge page for it so thought I'd give it its own thread here.

Erics Room Booking System.

A php/mysql driven booking system made for booking computer suites in the school here but could easily by adapted to book anything not just rooms.

Any number of rooms/assets.
Any number slots per day.
Booked by department/name/initials.
Uses can be local or authenticated from active directory.

SourceForge.net: ERBS