I second this. Very good tool for diagramming. All kinds of stencils / shapes / objects: Wireframes, UML, workflows, BPML,...

Edit: And of course you can draw free hand. It's vector based.

Around 2009 I saw someone's work made with OmniGraffle and it was so beautiful that wanted to have that too.

BTW, until recently OmniGraffle was the only commercial mac app I used in past 10 years. Now I got FL Studio as well.

+1