Fritzing

http://fritzing.org/ - open-source initiative to support designers, artists, researchers and hobbyists to work creatively with interactive electronics. We are creating a software and website in the spirit of Processing and Arduino, developing a tool 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.

Allows to model Arduinos, Pololus, and many other standard electronics items.