My fav pixel editor on PC is mtpaint -
http://mtpaint.sourceforge.net/download.html
side to this i use Gimp (particularly useful Dither where 12 bit color uniform color quantization can be performed) also as very important add-on constantly used by me is a Ximagic Color Quantizer
http://ximagic.com/q_index.html where SColorQ a.k.a. Spatial Color quantization can do miracles (even as a tool able to create psycho-visually optimal color palettes albeit non uniform from 12 bit color perspective).