Blackbody Radiation -- Check out the theoretical blackbody curve of unsuspecting stars of various temperatures from the comfort of your own home/dorm room.

Inverse Square Law -- This is a good demonstration of how the inverse square law works.

HR-Diagram -- This simulated Hertzsprung-Russell diagram shows you what happens to main sequence stars as they get old and start to die off. This is a good applet if you want to get a better feel how HR-diagrams work.

The Doppler Effect -- This is a great visual for how the Doppler Effect works.

Static Charges -- Play with static charges and see how the E-field and B-field change.

3D Static Charges -- Same simulation above...ONLY IN 3D! AWESOME!

EM Waves -- Amazing simulation that shows how waves emitted from a (or several) source(s) behaves in countless situations. You can see diffraction, refraction, reflection, interference. This simulation is great for understanding Optics.

3D EM Waves -- Very similar to the applet above (EM Waves), only this one is in 3 dimensions. Fancy!

Fresnel Diffraction -- We all love Fresnel Diffraction. You know, the diffraction that occurs when you pass monochromatic light through an apperture and it diffracts in the near-field. That's right. Now you can generate those patterns all by yourself without having to steal a key to the Optics lab.

Hydrogen Atom Wavefunction -- This applet displays the wavefunction of the Hydrogen atom in 3 dimensions. Pretty slick huh?

1-D Quantum Mechanics -- This is a quantum mechanics simulation that shows the behavior of a single particle in bound states in one dimension. It solves the Schrödinger equation and allows you to visualize the solutions.