Thanks J.Edward, that's a great site.

cgtextures has some great grunge masks that work well for making a floor look a bit dirty and broken up.
Here are some of the ones I use.
DecalsDamage0065_3_S.pngDecalsDamage0028_3_S.pngDecalsDamage0065_1_S.pngDecalsDamage0034_3_S.png