This is a workflow we've done for a couple of customers.  Let's say you want to ensure that a resource is not lying idle when a user cancels.  We have therefore created workflows which can pick up when a booking is cancelled, search recent bookings on that resource in a reasonable time period, extract the email address of each of the bookers and email them to say a slot has just become available.  This will happen automatically and is a useful way to nudge your users to potentially book and use the slot.