Fritzing

[Prototyping⇐] From prototype to product. fritzing.org

Fritzing is an open-source initiative
  to support designers, artists, researchers and hobbyists
  to work creatively with interactive electronics.

It is not a simulation package. It cannot be used to test a circuit.
It's use is for documenting a circuit and refining its physical layout.

Fritzing comes with a library of parts.
These are accurate representations of actual commercial parts.
You can also add your own parts to the library.
Specifically, it includes all the current Arduino family.

The Fritzing website has a large collection of sample projects.

Fritzing Videos