This file type plugin allows you to save images as 16 color bitmaps or gifs. Obvously full color photos will look terrible but simple logos and small grayscale images can look good (and save precious bytes ) . This plugin adds the options Bitmap (low color) and GIF (low color) to the save as type drop down. (and now png and tiff too)
Not massively useful but I wanted it and it was interesting developing them. A 16 color bitmap uses half the space for the image than a 256 color one (it also has a much smaller palette!). A 16 color gif is more efficient to compress with a lower number of colors.
UPDATE 04 Dec 2009
Preserve DPI setting when saving