Bruce Schneier and the Magic Water
It sounds like the Harry Potter series took a turn towards water sports, but it's actually security guru Bruce Schneier's take on Smart Water, a product you paint on your stuff to identify it as yours.
I think the best (unintended) use of this technology is to buy a ton of it and then dump it into a reservoir or large body of water: either way, it gets into the system (through regular water distribution channels, or evaporation and subsequent rain), and then you can claim the entire city as your own.
You say you washed your clothes this morning? Them clothes is mine now.