A toy I've been playing with is finally ready for a limited release. It's called Rowdy, and it's a classical particle simulation designed to be engaging and fun while giving users a good idea of how particles interact. It is not in any way rigorous.
The controls are currently quite limited. Z and X change the selected element; a left-click places new particles; a right-click removes the first particle in a one unit radius around the mouse (hold to remove many). F5 or R re-sets the playfield.
Pictured above is a "containment bottle" for the Elec particle (electrons, basically); some charged unmoving particles hold the charged moving particles (mostly) inside.
You can find downloads below; currently I can't build for Mac OS, sorry. The Linux binary requires SDL2 to be installed on the system.