These are a few of the OmniGraffle stencils I use often. I've been dragging them around in various forms for a while.
If you must have a license, I'll go with Artistic V2 as used in the Perl community. No warranty and all that.:w
Copy the Tobert.gstencil to ~/Library/Application Support/OmniGraffle/Stencils/Tobert.gstencil