When visiting Yahoo's site for OmniGraffle stencils, I found that Yahoo also provides a nice set of design patterns.
These Web/Web2.0 patterns are clear, concise and have pointers to how they are used within Yahoo. Note: this doesn't constitute a strong endorsement on my part. My approach to patterns is to broadly survey existing patterns with the goal of settling on one, or worst case to modify/create one using the results of the survey.
BTW OmniGraffle is an amazing drawing program -- I've been using simple/mid-level drawing programs for a while and its interface is in a different league entirely. It does things that I haven't seen before, hadn't thought of previously, but are totally obvious to use and highly useful. If you're on a Mac you owe it to yourself to give it a spin.