This may well qualify as a Drupal cheap trick. But it is pretty darn useful if you ask me.
The use case - I have a custom content type for daycares. In it, daycare owners can advertize the number of available spaces they have at any time. Creating a list of daycares showing available spaces is, of course, easy with Views. But what if I would like to show the total number of available spaces for the listed daycares. How would I do that?