I'm not a fan of 'traditional' toon outline shaders since they render the geometry a second time, just with black textures, inverted normals and a bigger scale to create an outline effect.

I like using a depthmap with simple edge detection (the outline is based on the depth difference between the shaded pixel and the pixels surrounding it)

Edit: Here's an example, what the technique that I've described (kind of) looks like:



but note that I've rendered that screenshot in blender, so the quality of the outline (especially the antialiasing) is higher than in most shaders.