Diffuse is a free diffusion filter for Photoshop, and anything else - like Paint.NET - that supports 8bf files (it won't run stand-alone).
"Diffuse" and "Soften" filters are available in just about every image editor, but this one supports sub-pixel, or fractional diffusion, producing smoother and more subtle effects (a "Diffuse vs Photoshop" image on the developer's site shows a comparison.)
Core operation is much like any other diffusion filter: drag a slider and a preview window updates to show any changes.
There's also an "Iterations" option which repeats the action anything up to 100 times.
A "Seed" value further randomises the results.
Diffuse is smarter than most diffusion filters, but you won't always notice the difference - it varies depending on the source image.