A (dirty) concrete mixer. I know it may sound slightly ungrateful and also a little inconsistent from my side, as it's been used to complete my house, but can't help thinking that concrete played one of the dirtiest parts in this world.
I thought at first it was lime scale on a domestic appliance (clearly far too familiar a sight in my house and a clear contender for the dirt of the world award).
I'm with Helen Jane and/or mixture on the cake mixers. They too are indirectly responsible for excesses of a different nature.