SPICE (Simulation Program with Integrated Circuit Emphasis) is a widely used tool for simulating electrical circuits. The original version was developed at the Electronics Research Laboratory of the University of California, Berkeley. Dozens of variations of SPICE exist, with both free and commercial versions available.
This site allows you to run ngspice simulations from a web browser environment. Whether you are using a Windows, Linux, or Mac computer, a smart phone, or a tablet, you always should be able to run a SPICE simulation!