At the Google Developer Day in Wembley Stadium the other day, I found out that the Google Visualisation API contains functions for parsing data returned from the Spreadsheets API, which is really awkward to parse by hand. I also got the callback working which catches notifications when an embedded YouTube video has finished playing. Combining these with a form for submitting rows to a spreadsheet, and you get the beginnings of a collaborative web playlist maker/player.

It only really works properly with YouTube videos so far, as they start playing automatically and send notifications when they're finished, but in theory any kind of embedded media could receive and send the same signals using window.postMessage.