I use CircuitLab, which is very helpful from a testing standpoint. As a coder, one of the things that really stresses me about electronics engineering is how untestable everything else, and CircuitLab gives me the ability to mock up simple unit-test-like circuits and see what the expected values should be, including when a power source fluctuates.

It costs a little, if that's a concern.

https://circuitlab.com