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In the same style as Processing and Arduino, “Fritzing” is an open source PCB layout tool that attempts to help designers move from physical prototypes to building actual products. In particular in the area of PCB prototyping, the freely downloadable cross-platform software allows for basic layout and design of projects like Arduino shields and other projects and can also be used to document PCB designs for later publishing on the web, etc… Lots of very good introductory information at the link below.

Fritzing – Link