Currency Graffiti is illegal, so of course there are websites devoted to such a wonderful cause. Does that make it art?
Throughout history, people have used currency to vent about virtually everything. From injustice & fighting the corporate power, to anything else they can think up. I have always been a big fan of graffiti as it's uncensored deep thoughts.
*FYI- For all those who don't solve geek math problems (I don't either), the answer to the equation written on the cheque of Mr. Randall Patrick Munroe is: $0.00 - You've been spanked! This dude is my hero!