Working with some images of maps, I had a need for some pixel grid overlays. I couldn't find any with a quick google search so I made these in paint.net using the Grid Maker plugin.
8x8 pixel grid on 256 pixel image
8x8 pixel grid on 1024 pixel image
16x16 pixel grid on 256 pixel image
16x16 pixel grid on 1024 pixel image
Hope this helps,