Pixatool (previously), an excellent app that turns any image into perfectly-tuned pixel art, is already on its second edition: a complete rewrite that adds a much better user interface, can batch-process images, and can load restrictive palettes for all your peculiar 8-bit nostalgia needs (I'll be making use of this to conform work to the Amstrad CPC pallette, one of the 1980s' more masochistic examples). Best of all, the new version's on sale at $9.95. 
The creator, Davit Masia, also created a simple online toy that turns pixel art into bead art or lego.
