I have been working with the Chicago Writers Conference to help build their conference registration, which launched this week. This was a fairly complicated process involving a lot of specific logic (you must register for x before you can register for y) and options that accommodated sessions with limited times, spots, and changing prices over time.
I spoke at this conference two years ago (about designing author sites) so it was fun to work with them in this capacity (a little) less scary than speaking in front of a room of people.