Refraction and reflection
http://www.walter-fendt.de/ph14e/refraction.htm
Good for reflection, refraction and total internal reflection (like in a fiber-optic)

Single Slit Diffraction
http://www.fys.kuleuven.ac.be/pradem/applets/suren/SnglSlt/SnglSltApp.html
http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/sinslitd.html#c1
Graphical Explanation

Single (screen) or a wavefront based one, and double slit diffraction interactive applets. See the interference of water waves.

Blackbody Radiation
http://webphysics.davidson.edu/alumni/MiLee/java/bb_mjl.htm
Shows the spectrum and resulting color (use IE on Mac)

Simple lenses and ray tracing
http://webphysics.davidson.edu/Applets/optics4/Intro.html
Can image lenses, various light sources and object

Electromagnetic Spectrum - see the EM spectrum, what the wavelength size is and what uses or generates the wavelength.

Absorption Applet

Emission Applet

Single Filter Applet

Fluorescence introduction, electron excitation and emission, Jablonski diagram

Human eye focusing

Human vision and color perception

Gecko images: red channel, green channel, blue channel

Bacterial colony image  

Breast cancer biopsy image