Wikipedia has an exhaustive list. In order to conversant in the graffiti world, you should be aware of these people:

Banksy is anonymous, but believed to be a British man named Robin Gunningham. He is a successful pop artist who does gallery shows. His hallmark is stencils of whimsical rats.

Shepard Fairey gained mainstream recognition for the Obama hope poster, but is best know for his OBEY campaign. It is the most iconic image in the graffiti world.

Marc Ecko is a fashion designer who has accumulated $100 million of personal wealth by producing graffiti themed clothing. His products are identifiable through the rhinoceros icon. He is also known for the 2006 video game Marc Ecko's Getting Up: Contents Under Pressure.