I know mosaic effects have been done before, but hopefully this one is different enough that someone will find it useful.
Uses path brush settings.
(Found under Artistic)
User Interface:
Control size: Sliders determine average size of tiles in pixels. Otherwise they determine how many tiles across and down. Tesserae across & Tesserae down: will also affect image due to changes in sampling. lock: when selected, the above sliders will be locked together. Tile irregularity: variation from square tiles to more irregular. Cement Thickness: pixel width between tiles. Cement Color: color between tiles. Reseed: generates new random tile shapes Blend Factors/Positions: These affect the path brush, Something to experiment with. AntiAlias: Does what it says, hopefully
Here's an example of a more abstract effect with different brush settings:
Tesserae.zip