Blek Le Rat
If you read my previous article, Top 10 Street Artists, then you'll know there is a wealth of talent out there on the streets. I came across a great article on RedBubble about some other awesome stencil artists. The article also includes links to tutorials elsewhere on the web.
Here are some of those other artists, plus an Instructables tutorial if you want to try your hand at it yourself...
Photograph to Stencil - More DIY How To Projects