One Plone project I am proud of is a small one for a local NGO - AKIfair - helping long term unemployed people with advice, guidance and courses. Also, they have good contacts to many regional companies willed to try to employ people with a more complex history.
The system is a database like application containing very very personal data including police records or medical data of the clients. There are employer records with job advertisements and a bunch of supporting records. All is internal only, behind VPN, well protected (but the network/protection is not my part here).
Today we added a custom made course based system with timetables. A course cycle is five weeks and each week one starts (except x-mas holidays). Courses and its lessons are generated rule-based, considering public holidays to take a different day and room for a lesson if needed. Attendees are added weekly to the 5 week duration courses. Attendance is tracked and if for some reason - like illness or taking an internship - the attendee need to be shifted to the next cycle this can be done with a few clicks. Timetables can be printed personalized for each client, trainer, or the room (room-management is included too).
Overall we will adjust details in the next weeks.
Implementation wise Plone really shines here. Technology used here are boring old school behaviors, views and one content-provider and some CSS, overall nothing fancy or special.
The only fancy thing are the rules, which are editable through-the-web. This was done using RestrictedPython very direct and very restricted to just a bunch of allowed functions.