A few months ago at the Google I/O conference, we were  approached by Zach Steindler, a co-founder at Olark  (a way to gain customer insight and sell better through live chat) who was raving about Google Analytics Annotations. He had such a great business case, we decided to let him rave here. Enjoy, and thanks Zach. 
You can annotate whatever you want; we annotate things like external publicity, major updates to our site, blog posts, even service issues, to see how all these events are impacting our business.Posted by Patricia Boswell, Google Analytics Team