Design Police is providing a "Visual Enforcement Kit" to "bring bad design to justice".

The kit can be used to print real-world stickers or as stencils.

How applicable the idea is can be discussed but the messsages include good tips (or real world problems) with humor.

More images can be found in the designer’s portfolio.

Dragonartz created a nice set of 8 free vector web stickers.

The set comes with 1 main EPS file and PNG files of every sticker.

There is also another set of vector stickers by Dragonartz which covers the different colours of the first circle sticker that can be found here.