There are few things I hate as much as grafitti. First of all, they damage other people's property, but also because most results are poorly made and meaningless.
But having said that, some are real artists and do really make a difference. Here are their websites:

• www.banksy.co.uk BANKSY is a pseudonymous England-based street artist, political activist and film director whose real name and identity remain unconfirmed and the subject of speculation.
• Julian BEEVER travels the world as an invited artist to create his three-dimensional drawings during art shows and street festivals
• www.blublu.com Produces by the street artist BLU, the edgy, scratchy drawing style requires erasing and creating countless images for this thoroughly engaging anmation
• Last two examples are via Jean Paul Lester's book Visual Communication, Images with Messages