Inkscape

Basic isometric terrain, floor and wall tiles with 45-degree slopes and internal borders, sized 64x64 pixels. Useful for building your own tilesets on top of them.

The tiles come pre-shaded so that the right side is always dark, but this can be easily edited.

Note that 45-degree tiles completely obscure their backward slopes, and so may not be suitable for all games, particularly games where you need to see these slopes and rotating the view frequently is not a desirable option. They are certainly useful for sandbox games where you do a lot of digging (Gnomoria uses them). To make the presence of backward slopes noticeable, create line sprites that are drawn onto the tops of the slopes and that have a distinct color (see pictures of slopes in the game Gnomoria for an example).

Tiles with 30-degree slopes are also available, and these don't obscure their rear slopes:
https://inkscape.org/~apmbcn/%E2%98%85two-level-isometric-tiles-with-30-degree-slopes

See the links in that

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