Transforming a smooth or progressive selection into a sharp one

Quick Mask is one of those underrated tools in Photoshop, which is a pitty because it can be so incredibly useful.  I also suspect that a lot of people miss out on both Quick Mask and Layer Masks by not realising that a lot of what can be done to an image layer, can also be done to a mask.

You could spend forever demonstrating this, so for a very quick example, Render > Clouds with a Reveal All layer mask selected produces a nice texture.  Of course you could get an identical effect using Render > Clouds to a layer with Red and Black selected as your foreground and background colours, but this effectively shows the flexability of adding effects to masks rather than layer contents.

I like to use levels or curves directly so you can control where the hard boundary is going to be, and keep some anti alias to avoid any jaggies.