SpriteStack.io is a tool for creating 3D pixel art using the technique known as Sprite stacking. Basically it’s a matter of creating a 3d object by making stacks of sprites. Sprite stacking is described as:
Sprite stacking is a simple technique that allows you to get 3D models by making a stack of 2D images. Each image is offset by 1 pixel and represents a horizontal slice of a 3D model.
The tool runs entirely in the browser and is very simple to learn. The help/documentation is available here. The tool is free to use but requires a $1 payment to enable exporting outside of the public gallery.