Processing is a programming language / IDE built on top of Java that’s intended for simple and visual programming, making it a great tool for WYSIWYS (what you see is what you simulate, actually that is not a real term, I just made it up).
If you’re unsure what Processing is, you might want to check out either the examples on processing.org or the double-pendulum I’ve just built into it. It is a port of my code for Arduino with TFT, but with Processing, it’s cleaner and way nicer to play with!
You can find an interactive version of my program here on openprocessing.org. It also includes a general implementation of the Runge-Kutta algorithm for solving ODEs, in case this is needed (without giving any guarantee of correctness or code quality – use on your own risk).
Update 2018-02-11: I’ve just published an updated version that simulates three pendulums in parallel with different algorithms (explicit euler, symplectic euler and runge-kutta). You can find it here.