Pixelslammer was a pixel-art tool I developed for use in Ludum Dare game jams - it was one of my first experiences programming with Python and Pyglet, and I have left it here in the hope of someday getting back to it and expanding it.
There are lots of tools that are good at this task, but many of them are showing their age. My personal favorite is TileStudio, which you can find at http://tilestudio.sourceforge.net/. It has the disadvantage that it only runs on Windows.