Any graphics app with a zoom feature would let you edit a picture pixel by pixel. I think one of the most popular shareware graphics apps is GraphicConverter.
Ah.... "pixelart"... that reminds me of the good old days... back when the iconfactory was pushing out 32x32 pixel works of art... I did quite a few myself..... and they looked so friggin' cool at the time. Then Mac OS X's beautiful 128x128 icons came around and screwed everything up.