OmniGraffle 7

Documenting and other designing can easily be done by this graphic making tool. It is available only for MAC OS. You can easily create flow charts, User Interfaces and some other diagrams. Whenever you need to make a quick sketch just go for Omnigraffle and it will not feel you sorry.

OmniGraffle Professional has everything included in Standard, plus features suited specifically for folks that make a living designing or working with graphics.
Source