The Stencilrevolution guys have beautiful galleries of stencils and their applications all over the planet. There's also a nice howto on how to do tshirts with stencils.

Somehow looking at such artful stuff tempts me to forget the stupid splashback tiles in the kitchen and try this: finish filling in the cracks, repaint with heavy white latex paint or similar and then do some stencilled spraying. Maybe some Escher icons on a sin() wave....or something like this?

I'm not a major fan of scribbly graffiti and tags, but the stuff presented there is mostly great art - and Banksy's rats are really cool.
More Banksy and non-banksy, both nasty and thus good: