From Credit Union Times an interesting slide show about the redesign of the $100 bill:
In October, the Federal Reserve will begin circulating a redesigned $100 bill. It’s only the fourth design for the note in nearly 100 years of circulation. Read more
The redesign of 1996 incorporated measures to deter counterfeiting, such as optically variable ink and hard-to-reproduce lines around Franklin and Independence Hall, microprinting, and a plastic security thread. The bills which will be put into circulation 10/8/13 were also designed to assist the visually impaired.