It wasn't intended to be used this way, but Topaz Vivacity, developed for noise removal, interpolation, JPEG repair and sharpening, can create some nice color-etch type effects when the sharpening settings are pushed to near the top end. Of course the effect can be used with any subject, especially those with contrast. Trees and leaves are especially nice. An added benefit is that Vivacity works well with soft or other slightly-off images that would otherwise be rejected. The main news today is that a Mac version was just released, a few years after the Windows version came out.