Circuit Simulator - Animated electronic circuit simulator using ideal components to visualize voltage and current.
GNS3 - GNS3 is a graphical network simulator that allows simulation of complex networks.
LTspice - LTspice® is a high performance SPICE simulation software, schematic capture and waveform viewer with enhancements and models for easing the simulation of analog circuits.
Cisco Packet Tracer - Download Cisco Packet Tracer, an innovative network simulation and visualization tool used by Cisco Networking Academy students and instructors.
Solve Elec - Solve Elec is a free educational program to draw and analyze electrical circuits.
Paessler Multi Server Simulator - Paessler Multi Server Simulator is inexpensive and robust software that helps you power massive-scale testing.