Electronic design automation (EDA or ECAD) is a category of software tools for designing electronic systems such as printed circuit boards.  In my professional life, I have been work apart from this domain, and only now because of renewed interest in electronics, I have come back to do some research.  I was pleasantly amazed by what I found.

Schematic and PCB Layout Editors

EAGLE (Easily Applicable Graphical Layout Editor) is a very popular (and free) solution for PCB design, including Schematic Capture, Board Layout and Autorouter.

• EAGLE
• Render 3D images
• Lady Ada Eagle library
• SparkFun Eagle Library
• DesignSpark PCB

Other tools are specifically focused on the Arduino that allows users to document their prototypes, share them with others, teach electronics in a classroom, and to create a PCB layout for professional manufacturing.

• Fritzing

Arduino Platform Simulation

• Arduino simulation and development environment
• Arduino Simulators

PCB Board Creation

You can crate printed circuit board (PCB) with the aid of some web sites.  You submit your PCB design; they add your design to the batch of orders. When the batch is big enough, the batch is set off to be manufactured.

• BatchPCB

SPICE

SPICE (Simulation Program with Integrated Circuit Emphasis) is a general-purpose, open source analog electronic circuit simulator

• LTspice IV
• LTspice Tutorial
• CircutLAB