Pixen is a sprite art program with many unique features. It is designed expressly for pixel art. To this end, it features extensive palette management and palette-swapping tools, a highly configurable grid overlay, a full complement of drawing tools, 'crosshairs' for ensuring proportion and straight lines, isometric line locking, extremely configurable backgrounds (for testing a sprite against various colors or images), and a full implementation of layers.
mtPaint is a simple GTK+1/2 painting program designed for creating icons and pixel-based artwork. It can edit indexed palette or 24 bit RGB images and offers basic painting and palette manipulation tools. Its main file format is PNG, although it can also handle JPEG, GIF, TIFF, BMP, TGA, XPM, and XBM files.
SVGSlice is a simple tool which generates chopped up images from Inkscape SVG drawings. To figure out where to cut, SVGSlice can use Inkscape's built in guides (for simpler, grid-like cuts), or it can use a specially named "slices" layer that contains rectangles that mark areas to slice. Demo files showing both approaches are included.
MapMemo allows you to drag your files onto any graphic (map, photo, scan, illustration, screenshot, etc.), and thus give them a visual, location-based relation. It uses only aliases of your files which, when you click on an appropriate location, will open in their original application. You can use layers, info windows, colors, and comment fields to organize your files on the map.