PhotoFrame Pro has been a staple in the Plug-Ins folder for many Photoshop users for a long time. The ability to create a frame effect that produced a mood or an attitude could make an image nearly speak.
LinkOptimizer is an InDesign script that processes any standard photos down to a set resolution.
Layer Groups adds two simple features to the standard layer options: Layer Groups (aka folders for organizing layers) and the ability to set layers or groups as nonprinting.
AKVIS Enhancer 3 is the latest upgrade of this software, with the main enhancements being a new interface and 16-bit image support. You can independently control the Shadows, Highlights, Level of Detail, and Lightness of an image.
Magma Effects is a clever plug-in for InDesign CS and CS2 that allows a number of Photoshop-like edits to be applied directly in InDesign.
If you like to experiment with designs and effects in Illustrator, you should check out these plug-ins.
Flash plug-in to move 2D objects in 3D space
After Effects plug-in to animate layers in 3D space
Powerful After Effects 3D graphing and animation plug-in set