A couple of really cool third-party AWS tracking sites have sprung up lately. Some of these sites make use of AWS data directly and others measure it using their own proprietary methodologies. I don't have any special insight in to the design or operation of these sites, but at first glance they appear to be reasonably accurate.
Tim Lossen's Cloud Exchange site tracks the price of EC2 Spot Instances over time and displays the accumulated data in graphical form, broken down by EC2 Region, Instance Type, and Operating System. Here's what it looks like:
The Spot History site also tracks the price of EC2 Spot Instances over time. This one doesn't break the prices down by Region. Here's what it looks like:
Timetric tracks the price of the EC2 Spot Instances and displays them in a number of ways including spot price as a percentage of the on-demand price and a bar chart. They also provide access to the for DIY charting.