While developing another project, I have created a simple demonstration of the additive blending mode in Flash.
Kirl used this to great effect in Wreckreation, and this isn't quite as cool, it does demonstrate the effect at it's most basic level. I do hope to recreate a similar fire effect Kirl made, but for now, if you have downloaded the attached ZIP file, please run the .SWF and try to drag the white circle.
This effect on it's own could be used to demonstrate how RGB works on colour computer monitors, to those who think primary colours are "Red, Blue and Yellow".