One of my favourite pastimes is to walk around the city for hours, and taking photos of wall murals, graffiti, even tags. There are some amazing street artists out there - you just have to keep your eyes open when you are walking around. I have found that Bushwick has some real gems, but Williamsburg isn't half bad either. I haven't found as many in Manhattan, but that's because I haven't walked around looking for a while either... Maybe today.
In any case here is my collection of photos as it is today... I will be adding more on an ongoing basis:
Street Art - NYC