Is there a way to add a black outline to sprites?

[Crunchynut44]

Has this been done before? It would be a cool look, I thought it was effective in Borderlands.

[Albertoni]

Action Doom has some of that cel-shaded look. As for sprites, I don't think you can do that, other than editing them one by one or writing a script to do that.

[Revae]

If you wanted to make an action in photoshop for it you could attempt to set it up to duplicate the base layer, find edges filter, desaturate, adjust levels, and multiply it over the base layer.  Looks like:

I'm sure you could tweak the method.  If you just want a black outline around the sides only, just add a stroke (actually don't use stroke, use an opaque drop shadow with a 98% curve to avoid anti-aliased edges).

If you want to do it with 3D models like action doom did: duplicate the model, select all vertices and move them along their local z axis (inflates the model like a balloon), then invert normals, and apply a black texture.

If you want to do it with a shader: ask someone else (not that you asked me).