Select Edges for Mac

From Reindeer Graphics, Inc.:

Select Edges is designed to easily make a selection using one of five different edge finding algorithms. These selections can then be used to smooth or sharpen images, a powerful Photoshop technique. The big advantages of Select Edges are that it works in one step, it offers multiple types of edge detection, and it functions in Grayscale, RGB, Lab, and CMYK color spaces, both for 8 and 16 bit images.