Box Fitting is a free filter for Photoshop, and anything else - like Paint.NET - that supports 8bf files (it won't run stand-alone),

Give the filter a source image and it replaces the original pixels with randomly fitted boxes of various sizes, like a more abstract and interesting pixelated or mosaic effect.

The results can be tweaked by dragging sliders to set the maximum size of each box (larger = more abstract), the maximum number of boxes, and a random seed which varies the results.

By default Box Fitting samples its colours from the image, but there's also an option to set a custom colour and variation threshold, which might produce good results with the right source.

Verdict

A interesting and more abstract variation on the standard pixelate effect.