You OmniGraffle fans probably already know about Graffletopia, which hosts a tons of cool stencils for your downloading pleasure. We've also got quite a few linked from the OmniGraffle extras page, and of course it's super easy to make your own stencils by selecting New Stencil from the Stencils menu and dragging in images.
Currently we aren't including very many ready-made stencils in the app itself, though, so we're going to be working on adding more Omni-created stencils to Graffle's built-in library of objects (and updating a few of the existing ones).
Here's where you can help: what sorts of image collections would you like to see in the app? I am personally voting for a Macworld booth layout stencil (complete with $100-per-day rented trash can!), some cool photo borders, and maybe some DVD/DVR/remote icons (so when my in-laws visit we can create a foolproof graphical set of instructions to help them operate the television without accidentally resetting everything to Spanish, adding a season recording of “Deal Or No Deal?”, and turning the screen magenta), but I figure you guys might have some ideas that are even MORE useful.
Post your stencil requests in the comments, and thanks for helping us out!