20-Sim
20-sim, the dynamic modeling and simulation package for iconic diagram, bond graph, block diagram and equation models
http://www.20sim.com/
Expertune
Expertune
http://www.expertune.com/
Matlab
The mathworks is the world‘s leading developer of technical computing software for engineers and scientists in industry, government, and education.
http://www.matlab.com/
National Instruments
Ni leverages commercial technologies, such as industry-standard computers and the internet, to deliver customer-defined automation, test and measurement hardware and software solutions for engineers and scientists.
http://www.ni.com/
Simulating circuits
Free source of spice circuit files and tutorials for the hands-on learning of electronics through the spice simulator.
also inlcudes spice basics and a command summary. links to free demo versions.
http://www.ecircuitcenter.com/