DrawExpress is a gesture-recognition diagramming application that lets you draw diagrams and flowcharts, including use case, sequence, class, state, ER, data flow, and network diagrams. Beyond UML and network diagrams, you can diagram business processes and mind maps, including flowcharts, organization charts, process flow diagrams, feature lists, decision trees, and visual notes. By "gesture recognition," the DrawExpress people mean that the UI recognizes your sketched objects instead of making you pick them from menus.
A free DrawExpress Lite version lets you decide whether DrawExpress is right for you before paying for the full version. If it could import and export Visio objects and diagrams, it would be even more useful. I recommend using a stylus with this app.
DrawExpress Diagram, DrawExpress, $5.99, free lite version