iServ is a web application for booking shared resources (i.e. meeting rooms or projectors). It has a simple browse and click user interface and a fairly complete administration section.
In addition to being open source (thus freely modifiable), can be mentioned these features:
Access to bookable resources can be controlled on user or group level. Its also possible to define number of repeating booking or how long time in advance a booking can be made. Bookable areas/objects can be locked (for new bookings), optional with a redirect page or popup info.
iServ has been tested to run on UNIX/Linux (recommended) or Windows. Installing to Windows is not a task that is being thoroughly tested. Mac OS X and other UNIX like system should probably also work (see requirements).