This site is a huge collection of simulation applets for analog circuit, filters, acoustics, general harmonic stuff, quantum mechanics, thermodynamics, linear algebra, and so on.

The analog circuit simulator in particular is very feature rich. I have used it to design some synth circuits in lieu of/addition to breadboarding.

Also try the acoustics ones, especially the ripple tank. The examples drop down has a bunch of cool setups like speaker designs, mirrors, lenses, mechanical filters(!), and can even simulate temperature (impedance) gradients.