From the Google Code site, the Periodic Table of Google APIs & Developer Tools is a cool layout of the tools available.  It’s actually well designed table, so each element is clickable, and takes you to the information page about that particular API.

They’re color-coded by category, but many of them belong to multiple categories.  For example, the Google Analytics is part of Data APIs, Ads and Tools.  If you mouse over the category names at the top, all of the members of that category are highlighted below.

Found on Twitter through @illuminantceo