Custom Halftone Pattern in Photoshop

You can simulate the halftone process in Photoshop by using the hard mix blend mode.

Create a custom halftone screen pattern of any kind you like. Use continuous tone grays from black to white. When you combine it with a black and white photograph using the hard mix blend mode, the grays will interact to force each other to black or white in the appropriate proportions.

A blurred checkerboard makes a nice basis for a halftone screen pattern.

I used twirl on the halftone screen before hard mixing it with the photo.

A hard mix with 50% gray is pixel-identical to a threshold adjustment layer at 127. You can perfectly duplicate the behavior of the threshold adjustment layer by varying the gray value with which you hard mix a photo layer.

UPDATE 6/13/2009

Use the hard mix blend mode to combine a black and white rendering of a 3D object with the same object texture mapped in closely-spaced wrapping stripes or dots and you'd have a halfway decent stab at rudimentary non-photorealistic digital engraving.

UPDATE 6/21/2009

Stochastic Halftones using the Photoshop Dissolve Blend Mode