Here are math and physics ones: https://falstad.com/mathphysics.html
I like the 2D vector field one https://falstad.com/vector (2d) and the 3d one https://falstad.com/vector3d.
Antenna simulator: https://falstad.com/antenna
Waveguide is awesome too https://falstad.com/embox/guide.html. Don't forget to pick various modes in the little square at the bottom.
If Paul Falstad comes around these parts, thank you for creating and sharing those!