gplEDA.org
Fritzing forum

Fritzing is essentially an Electronic Design Automation software with a low entry barrier, suited for the needs of designers and artists. It uses the metaphor of the breadboard, so that it is easy to transfer your hardware sketch to the software. From there it is possible to create PCB layouts for turning it into a robust PCB yourself or by help of a manufacturer.
gEDA Project wiki | git | pcb | gerbv | symbols | icarus | gtkwave | help | user

The gEDA project has produced a GPL'd toolkit of Electronic Design Automation tools. These tools are used for electrical circuit design, schematic capture, simulation, prototyping, and production. The gEDA project offers a mature suite of free software applications for electronics design, including schematic capture, attribute management, bill of materials (BOM) generation, netlisting into over 20 netlist formats, analog and digital simulation, and printed circuit board (PCB) layout.
KiCad users | developers

KiCad is an open source software suite for electronic design automation (EDA). It facilitates the design of schematics for electronic circuits and their conversion to PCBs (printed circuit board) design.
Qucs help | devel

Qucs is an integrated circuit simulator which means you are able to setup a circuit with a graphical user interface (GUI) and simulate the large-signal, small-signal and noise behaviour of the circuit. After that simulation has finished you can view the simulation results on a presentation page or window.
XCircuit dev

XCircuit is a UNIX/X11 (and Windows, if you have an X-Server running, or Windows API, if not) program for drawing publishable-quality electrical circuit schematic diagrams and related figures, and produce circuit netlists through schematic capture. XCircuit regards circuits as inherently hierarchical, and writes both hierarchical PostScript output and hierarchical SPICE netlists. Circuit components are saved in and retrieved from libraries which are fully editable. XCircuit does not separate artistic expression from circuit drawing; it maintains flexiblity in style without compromising the power of schematic capture.