Circuit simulation 

is a technique of predicting the behavior of a real circuit by a computer program. It replaces real components with idealized electrical models. Since the result can never take into account all physical processes in the parts and all PCB parasitics, it will only reflect the model that is put into it. Therefore the simulators can't substitute breadboarding and prototyping. On the other hand, they allow measurements of internal currents, voltages and power that in many cases are virtually impossible to do any other way. Most general-purpose program are based on various versions of SPICE (Simulation Program with Integrated Circuits Emphasis) and can handle analog, logic and mixed-signal components. Commercial software usually include schematic capture, extensive model libraries, and waveform display and analysis functions.

Electrical design and analysis programs are relatively expensive. Designers who want to simulate their circuits without buying a full package have basically three options: 1) Working demos (usually limited to small-size circuits with some functions disabled); 2) Open source programs (with little or no tech support); 3) Software provided by ICs manufacturers (normally good only for the designs that use these manufacturers' parts).

Here you will find links to free electronic circuit simulation software, power supply calculators, and other electrical engineering design tools on the web.