OmniGraffle has helped me make sense of my thoughts into a diagram format. It has become a visual language that communicates information quickly and effectively. The program allows you to change the colors, shapes, styles, and text attributes on elements in real time without having to rework the whole thing. You can even collaborate with other people on a project using the built-in sharing options. It works great when working with others on a single project or collaborating on multiple projects at once. It's really helpful when designing for mobile because it gives you an accurate representation of how the final design will look. You can make a prototype of the entire layout on paper, move it around and test out different scenarios before committing to a specific layout on screen. I love OmniGraffle's "Timeline" feature. You can view the past versions of a document so you can see exactly what was changed each time. This helps tremendously when designing a UI for an app, since it allows you to keep track of exactly what components were added and removed along the way. You can save an image to any file type including SVG, PDF, EPS, PSD, PNG, GIF, JPG, TIFF, etc.