The Null Device
A recent Worth1000 contest involved making Photoshopped versions of M.C. Escher images. There are some really good ones here.
Sonic Finger sell some interesting audio plug-ins, including the Dead Quietenator, which provides 56 types of digitally-modelled silence, including previously unattainable vintage sileces, and Virtual Studio Visitor, which, when applied to a track, simulates the effect of a specific visitor watching the performer (presets include "Guy From Label", "Resentful Girlfriend/Wife", as well as generics like "Clown" and "Ninja"):