One of the reasons for Photoshop's popularity is the huge number of third-party plug-in filters available for it. The chances are that whatever effect you want to achieve, there's a plug-in that can do it, or one is in development. The downside to this constant supply of impressive effects is that Photoshop's standard filters get overlooked, especially innocuous-looking ones like High Pass, a well-kept secret in the backwaters of the Others sub-menu.
At first sight, High Pass appears fairly mundane - all it seems to do is flatten and desaturate an image. But once you understand how it operates you'll appreciate the tool's extreme versatility. It's part of a family of filters that enables you to manipulate the different frequency ranges of information that compose an image.