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.
Logicly - - Design circuits quickly and easily with a modern and intuitive user interface with drag-and-drop...
simavr - a lean and mean Atmel AVR simulator for linux
Digital - A simulator for digital circuits.
SimulAVR - The SimulAVR program is a simulator for the Atmel AVR family of microcontrollers.