Mark McGowan is an artist, and his canvas is other people's cars. His paintbrush is a key. He goes around town scratching his 'artwork' on random people's cars. At least, that's what he claimed. Now he's admitted the whole thing is a hoax:

An artist who claimed to have vandalised nearly 50 cars in the name of art has admitted it was a stunt. Mark McGowan, 37, said he had taken pictures of himself scratching vehicles' paintwork in Glasgow and London as part of a project. But the London performance artist has since admitted the cars were already "keyed" and the photos were "staged". He said it had been an art project that had gone "horribly wrong" and said he was "very, very sorry".
"I never keyed any cars...the whole thing has just been a nightmare," he added.

I actually think it's a pretty clever idea for a hoax. It's so outrageous, and yet at the same time easy to believe an artist might actually do this. Some of McGowan's other stunts also sound, um, interesting:

In 2003, he used his nose to push a monkey nut seven miles to 10 Downing Street in protest against student debt. He also rolled on pavements for four miles across London singing Christmas carols to highlight the work of office cleaners.

(Thanks to Andrew for the link)