What OS X app can I use for wireframing? I'm looking for something to replace Visio on a Mac.  I am getting tired of using VMware because it's too slow on my computer and was wondering if there's anything out there I can use to wireframe apps specific to Macs.
If money isn't an issue, or one that you can wrestle with, then I don't think there is really any better tool for this than OmniGraffle by the Omni Group. It doesn't come with interface elements by default, but there is a freely-available extensive library of options on their site Graffletopia.