I really like this one, for some strange reason:

https://www.falstad.com/embox/

>"This java applet demonstrates standing electromagnetic waves in a 3-d rectangular box, or cavity resonator."

Maybe it's the idea that waves can carry slower frequency secondary waves, the idea that fields can carry slower frequency secondary fields, the idea that EM space itself can be stressed in a number of dimensions, and that these stresses, (depending on waveforms and modes of oscillations) could give rise to even smaller regions of stronger, localized (yet time/period/frequency dependent!) stress... much like waves in the ocean... to use a simple analogy... but this app really shows us what's possible...

Anyway, this page of apps is a must-have resource for any Physics or Engineering student... A+++++ !