Its purpose is to enable you to create non-dithering patterns, so that you remain inside the 216 color limit, and it works quite well, with the possibility of saving your settings etc.
Usefulness :
GUI :
Image Vice
Helps you compress the image by reducing the size of the palette, with a few settings to play with, but although it is quite simple to use it remains primitive and not-so-efficient.
Usefulness :
GUI :
PhotoGif
One of the few plugins designed for an efficient and precise color management when turning your nice, true-color *.psd images into these horrible 216-color Gifs... You begin by creating the color scheme you want to keep with the help of the plugin, then apply it to your image, with palette options, the possibility of saving/loading settings, etc.
A really great utility, that leaves an alternative to the painful process of creating non-dithered patterns.
Usefulness :
GUI :
ProJPEG 3
OK, only one more JPG plugin for Photoshop (about 10 for the moment), this one is very simple so you can use it without spending hours on the manual, its main advantages being a very fast preview as you adjust settings, which is great with big images, and the possibility to save presets.