EveryCircuit Free is an interesting Android app that aims to help to understand how electronic circuits work. Build any circuit, tap play button, and watch dynamic voltage, current, and charge animations. This gives you insight into circuit operation like no equation does to most of the people. The software is easier to start to use than many well known SPICE tools (like Multisim, LTspice, OrCad or PSpice). Surprisingly beautiful and impressive work. EveryCircuit is possibly the best circuit design and simulation app on the Android market at the moment.
Free version has limited sandbox area, but it can still impress you. The software contains good circuit examples to refresh memory of basic principles and animations show quickly how things work. The small sandbox area can quickly annoy you on this free teaser version, meaning you can only use the smallest example circuits. If you need a larger playground, a full version of EveryCircuit is available for some money.