Locking for a global Calender

I need either some free or cheap software that is a global calender for a small peer-to-peer buiness.

It needs to be accessable by many user from any machine and be edited and read by all users.

For example, John ask for holidays In Jan the accounts lady puts john as being on holidays in the calender so if some one rings up to as the service manger if they couldbook john in to do some work in jan thenhe could check and see that he wasnt in. This needs to be for multipal users

Thansk..