He also made circuitjs, a very useful circuit simulator which I regularly use for prototyping. It lets you modify and probe elements in real time unlike spice simulators, so it is faster to modify a design in it.

I use spice simulators to check the final design since sometimes circuitjs can have instability issues in complex circuits involving stuff like the nonideal op amp.

https://www.falstad.com/circuit/circuitjs.html