A Handy Reference For Arduino Shields
If you’re using an Arduino in your projects, sooner or later you’re going to want to do something more than the standard hardware is capable of. Then you need a shield, a secondary circuit board that plugs into the main Arduino board. For help finding and using these shields, check out the Arduino Shield List, a project in progress from a member of the Arduino community. And for help with your next DIY project, check out this guide to Arduino.
Via Arduino blog
A Guide To The World Of Arduino
Everything you need to know about Arduino.