To quickly visualize an architecture or a structure Visio might be the tool of choice. But to make such a diagram really appealing you have to have the right shapes.
To create diagrams for SharePoint there are a couple of free shapes available. I collected some of these shapes:
- MOSS (2007) Site Strukturen
- Visio Stencils for IT Pro Posters (SharePoint 2007)
- SharePoint Visio Stencils (SharePoint 2010)