Sketches are often better tools for communicating designs than high fidelity illustrations or schematics. The purpose of the sketch style wireframe is to prevent the intended audience from thinking about visual design and focus on the functionality and behavior you are proposing.
I'm happy to announce the release of this set of sketch stencils for OmniGraffle, based on the Wireframe Stencil. This set of user interface sketch stencils is designed for OmniGraffle 5.x. OG4 users are able to use the stencils by changing the .gstencil extensions to .graffle. The sketch kit contains all of the UI elements you'll find in the free Wireframe Stencils we provide at Konigi, plus additional shapes and elements to make it easier for you to adjust to this style of wireframing.
As usual, I'm sticking to the goal of keeping the tools on this site inexpensive or free. Your $10 purchase of this stencil set entitles you to free upgrades forever.