EAGLE stands for Easily Applicable Graphical Layout Editor. This software offers user friendly, powerful and affordable solutions for PCB design, including Schematic Capture, Board Layout and Autorouter. User Language Programs enable features such as simulation and 3D-visualization. There is a free version of the EAGLE PCB editor, with some features disabled.
Fritzing is an open-source initiative to support designers, artists, researchers and hobbyists to take the step from physical prototyping to actual product. We are creating this software in the spirit of Processing and Arduino, developing a tool that allows users to document their Arduino and other electronic-based prototypes, and to create a PCB layout for manufacturing. The complementing website helps users to share and discuss drafts and experiences as well as to reduce manufacturing costs.
Arduino is an open-source electronics prototyping platform based on flexible, easy-to-use hardware and software. It’s intended for artists, designers, hobbyists, and anyone interested in creating interactive objects or environments. There is a wealth of resources on their web-site: tutorials, examples, code libraries and a very active community in the forums.