SNG enables you to decompile PNGs into an information-preserving .sng text format that you can hack with a text editor. Then you can recompile the .sng back to a .png -- handy if you need to add or edit some obscure ancillary PNG chunk that your graphics tools don't understand. Also handy if you want to generate PNGs programmatically using text tools.
Sticker Book is a children's program which lets you place stickers on a background scene. You can resize, recolour and reorient the stickers that you use, and you never run out. You can even paint with them. The program is intended for the 3-7 year olds, but it's even fun for adults. It is flexible enough to make sophisticated pictures without being as complex as drawing/painting program.
SunSky uses the Utah sun/sky model to draw plots of the entire sky, or a specific window of the sky, for a given location, time, and view direction. You can also vary the turbidity, which controls the level of haze in the atmosphere. (Los Angeles would have an extremely high turbidity, Greenland would have an extremely low one.) A little tweaking can quickly lead to a picture of the glorious sunset you'd be seeing outside if it wasn't currently pouring with rain. SunSky can also draw the CIE colour diagram, and various spectra, including blackbody radiation and the CIE basis functions.