PartSim
PartSim is a free and full-featured SPICE circuit simulator that runs directly in your browser. PartSim schematic editor runs in your browser using javascript and does not require the use of any plugins. All you need to do is draw your schematic using components that are available in the tool's sidebar, place your desired current and/or voltage probes, and then click the "Run" button. This will bring up a simulation dialog box that allows you to run various simulation types (DC, AC, Transient). PartSim uses a cloud-based version of ngspice, which is an open source simulator based on Berkely's Spice3f5 Spice engine.
