for me, the most interesting project I''ve worked on was 4 years ago for a beer company.
it was a facebook app developed in HTML (not html5), jQuery & jQuery hi, php, imagick, ffmpeg, & YouTube library.
for the Euro Cup, users had elements to drag and drop on a stage, add frames, dialog boxes, and create a 15 second animated story board. all positions of these elements along with the frames where sent server side to create images of each frame (rendering fronts and positioning), then combining them using ffmpeg to generate a video.
these videos were later uploaded on the client's YouTube channel.

this project was awesome, knowing css3 and html5 were prohibited to use due to cross browser compatibility. it was ban exercise on all levels :)