It depends what you're doing - for analogue circuits the free software LTSpice is pretty great. It's clunky and you need to follow some tutorials, but it's a br

It depends what you're doing - for analogue circuits the free software LTSpice is pretty great. It's clunky and you need to follow some tutorials, but it's a brilliant way to sanity check an analogue design before you build it. You can set square wave voltages to see dynamic performance. If you want to simulate logic then logisim is really useful - you can design your circuit in terms of logic gates and then change inputs to see how it behaves - or generate a truth table. Good for checking that your logic works as expected.