GIMP with several thousand isometric sprite brushes from various games and mods, along with terrain textures and tool presets to easily make nice maps for your campaigns. Many of the brushes contain multiple images so you can conveniently spread common objects that are not all clones of each other. I have made some additional sprites so that the set covers all the needs I can think of. With some practice and knowledge of the sprite sets you could make usable maps on the fly if you really needed to.
The sprite sets from different games are not optimized to work with each other perfectly. The most saturated colors have been toned down, but restrain is required when it comes to mixing styles.
Obviously not to be used for anything commercial. If you publish your maps anywhere, giving credit to the source game(s) would be polite.
Illustrations from various RPG books made into printable black & white miniatures.