TinyCAD

Available on Sourceforge, TinyCAD is an open-source program for drawing electrical circuit diagrams, or schematic drawings. TinyCAD is a PCB design software tool of choice for electronics engineers who want a solution that supports standard and custom symbol libraries and supports PCB layout programs with several netlist formats.

Key Features:

Often used to draw one-line diagrams, block diagrams, and presentation drawings
Flat or hierarchical schematic entry
Includes many popular PCB layout-compatible netlist formats
Convenient support for embedding graphical images in drawings