CircuitLab

CircuitLab is a web based circuit prototyping tool. It allows you to quickly and easily design and test circuits to identify problems before breadboarding your design. You can also easily share your designs by exporting schematics as images or PDFs and linking directly to your CircuitLab designs.

Features

 * Free (for now)
 * Browser based
 * Allows you save multiple designs
 * Allows you to simulate your designs so you can debug them before building any physical circuits
 * Easy to share designs without any risk of someone overwriting your design
 * Nothing to install
 * Generates nice schematics that can be saved and shared online or offline
 * Works on any operating system

Disadvantages

 * Can't import from other design tools yet
 * Can't export to other design tools yet
 * Touch-based browsers are not supported yet
 * Chrome or Firefox recommended
 * May require payment for advanced features later