Wikinvest.com [wikinvest.com] has a feature called Wikicharts which allows users to share annotations on financial charts (e.g. Apple stock, Google stock). The annotations are displayed in a similar manner to the ones used to indicate news stories (and more) on Google Finance, however they can also span ranges to help explain changes over time.
Google's chart is well respected and has inspired at least one credible open source imitation, although wikinvest's version is the first I've seen that exceeds Google's features while achieving the same quality. I'm sure there are many more to come, provided people pay attention to the small details that make it a responsive interactive experience.
The idea of graphical annotation of charts is one of the concepts explored in the sense.us project which fed into the Minnesota employment explorer, some of the work at Many Eyes and the newly-launched New York Times' Visualization Lab. It seems only a matter of time until tools like these converge with software such as Skitch to allow contextual annotation, embedding and sharing for all online content.
Guest blogger Tom Carden is an interaction designer at Stamen Design. He has recently contributed to several successful visualization projects including MSNBC's Hurricane Tracker, Trulia Snapshot and mySociety's travel time maps.