The spec documents showed video being played from a thumbnail image in a pop-up video player. When we started the site, they only had vimeo versions of the videos, but we knew that we were eventually going to want to move to flowplayer video for the final 'go-live' version of the site. I built the block to rely on other built in and marketplace blocks to display it's video from any one of Vimeo, YouTube or FlowPlayer videos.
This made for the most complicated block control interface I'd ever programmed. There's a lot of interactivity programmed on the buttons and widgets to make sure that saving validates properly and that the interface works as expected. It was a great learning experience.
There were several changes in the eventual marketplace release of this block, but Welcyon is where it was first created. It's been a pretty good seller with pretty decent reviews, people really seem to like it.