Pixel by using a computer is cool, but the idea of putting blocks on a raster is an old childgame! For 40 years a small company sells a game, which is called ministeck. Check the company page www.ministeck.de to get an idea of the purest way of pixeling or check the ministeck simulator to ministeck online.
yey, ministeck.. only crit on this stuff is that it's more repixeling preassembled horses or other postcard-motives (that are delivered in the carton) than pixeling something own..
nevertheless: http://www.piexeln.de/pics/tomic-evoke2005ministeck.jpg
Hah, I had something like this when I was a kid..
It has more to do with tetris than with pixelart IMO tho :P