I offer for your consideration this wireframe stencil for OmniGraffle Professional. I’ve been using it and finding it useful for the last few months. Download it here:

Whale Wireframes.gstencil (881Kb .zip)

It’s important to note that this is meant for the Pro version of Omnigraffle, with table support. It includes a dozen custom-made shapes, the kind that can only be added to OmniGraffle by writing XML code.
These shapes are mostly rectangles with stroke only on three sides, and they’re built so that the unstroked side does not curve when using corner radius:

Using these shapes inside tables, one can build interesting interface elements, such as button bars, progress bars, etc. Each of the composite items below acts as a single table object in OGP:

I hope you’ll find these useful. I’m always working on this wireframe kit, so stay tuned for updates, and let me know what you think.