pngquant is a batch conversion utility to quantize and dither truecolor PNG images, especially those with a full alpha channel, down to "RGBA-palette" PNGs (i.e., PLTE + tRNS in PNG parlance). Such images are usually two to four times smaller than the full 32-bit versions, and partial transparency is preserved quite nicely. This makes pngquant especially useful both for Web sites and for PlayStation 2 development, where one of the texture formats is RGBA-palette-based (though not PNG-compressed).
Toolbar Icons is a collection of free toolbar icons. All icons are available as (Inkscape) SVG files. OMake build scripts that automatically generate PNG files of basically any size (using Inkscape) are included. PNG files in a few default sizes (16 x 16, 24 x 24, 32 x 32, and 256 x 256) are prepared in .tar.gz archives.
HGL is a compiler/interpreter suite for developing images. It features its own simple but powerful language, an output format configurable by plugins, runtime input handled by plugins, and easy integration into various environments like Web servers or graphical applications. The input is taken from a source file, which has to be compiled for quick and frequent access by the interpreter. An interpreter then runs the compiled files, takes input from custom plugins (if neccessary), and outputs its result via custom plugins.