If you use a Mac, you would probably use OmniGraffle to replace Visio as a tool to create diagrams, flowcharts and other similar documents. The coolest thing with it is you can add a list of graphics called stencils from the originally provided ones to make a wider use of the app.
Just today I stumbled on Graffeltopia, a site dedicated to give OmniGraffle users a huge range of free stencil add-ons. These are contributed by users, so expect for the number to keep on growing.
Check it now to start adding your stencils or look for the ones you need for your projects.