Fluorite Lens
From my own personal experience, this fluorite element(s) can really bring some magic into the photos.
Here is a old list of the lens with fluorite element
And more information about it can be found in Canon's Lens Work (p172), online version here
http://software.canon-europe.com/files/documents/EF_Lens_Work_Book_9_EN.pdf
New EF 800/5.6L IS have two fluorite elements in it!