Google is a behemoth with a massive amount of APIs that millions of developers use around the world to build millions of apps and services. Since, there's no simply way to keep track of them all, Google has a pretty nifty "periodic table.
As you can see in the screenshot, Google has a full periodic table built out showcasing their APIs and developer products as of January 2011.
Along the top are various categories that the APIs or products fall under. Included are: Mobile, Search, Gadgets, Data APIs, Social, Misc, Ads, Geo, Tools, and Chrome. Hovering over any of these will highlight the corresponding "elements"on the table below it. You can then click on any of the elements, and it will take you to the page dedicated to that API or product.
More Info: Google APIs Table