Gliffy Diagrams puts a variety of business chart styles at your fingertips. The app can create business process modeling, flowcharts, entity relationship and BPMN charts, among others. There's even a section for mocking up a user interface.
Just select the category you want to work with, grab a shape, a text box or other item and place it on the interface's grid background. Add more items and connect them with lines. The app includes a good assortment of geometric shapes and standard flowchart symbols, standardized BPMN shapes and Venn diagram circles in a variety of colors.
In addition, individual items can be grouped and snapped to a grid. However, you can neither color nor rotate them.
When you're done, the chart can be exported as an image.