Client waiting list (Wait List)

It would be great to have a list that we could add clients to that would like an appointment if we get a cancellation.  If a cancellation occurs, the list pops up and we can give that client a call to let them know we just had a space open up.  Maybe even by time, so if we have a two hour client, and a one hour appointment open up, it doesn't pop up.  Only for the time that we need?  But either way, a client waiting list would be great.