Google recently added a new feature on Analytics - graph annotations. It's a small feature, and I had to do a double-take to make sure I wasn't seeing things when I first saw it. You can click on any date in the graph and make a note that you can make private or share. Below the traffic graph, there's an ajax arrow that you can click on to make annotations or see a list of them. It looks like the feature only supports notes on a day in the graph and not a range of dates. It's a helpful new feature. If you launched something new on your site, you can annotate it to see how it affects traffic. If you had a traffic spike or dip, you can find the source and note it on the graph. It's a simple little feature that's great for keeping track of traffic-related things.