Our Yacht Club client has three workslips that members can reserve for up to 3 days. Using core taxonomy, content types, views, and the Events, Conditions, and Actions (ECA) module, we built a system to allow members to make reservations, and have the system prevent double-bookings.
One of our clients is an art gallery that has a daily light show with limited seating. Reservations are free, but the show is popular enough that you need to reserve a month out.
Each day, they print a list of reservations for the staff to use when admitting guests.
Another automation we did for Programming Librarian, a site for librarians to plan educational programs, involved events. They wanted to always feature 3 events on the home page, and the most important events were in the future. If their schedule is full, they wanted 2 future and 1 past event visible -- but they don't always have upcoming events, so there might be 0, 1 or 2 future events, and 3, 2, or 1 past events.
Views module has long been the killer feature of Drupal, making it easy for a site builder or skilled administrator to essentially create complex SQL queries through a web interface, without knowing SQL.
