Logisim - Logisim is an educational tool for designing and simulating digital logic circuits.
Xyce - Xyce is an open source, SPICE-compatible, high-performance analog circuit simulator, capable of solving extremely large circuit problems by supporting large-scale parallel computing platforms.
Digital - A simulator for digital circuits.
simavr - a lean and mean Atmel AVR simulator for linux
Circuit Simulator - Animated electronic circuit simulator using ideal components to visualize voltage and current.
SimulAVR - The SimulAVR program is a simulator for the Atmel AVR family of microcontrollers.