Runway is an elegant UML designer that is both simple and powerful. It gives you all of the most important features you need in a diagramming and vector-design tool, as well as an easy-to-use class-diagram tool without all the extra cruft, at an affordable price.
Runway is great for programmers and Web designers looking for a simple tool for rapidly designing flowcharts, laying out wireframes, or visualizing model relationships without getting lost in a sea of confusing and extraneous features. Runway has support for "connected" lines. When resizing lines, the lines magnetically snap to other shapes. Then, as you drag the connected shape around the canvas, the connected lines adjust themselves as well. Runway offers "smart" paths which can be used for connecting boxes and shapes with smooth, curvy lines without requiring lots of fussy fiddling. Runway also includes a powerful Bézier path tool for creating vector graphics of nearly any shape or description. Runway is a swell vector graphics tool for designing icons and other graphics using Bézier paths.
Finally, when you're all done, you can export your compositions in common image formats like PNG, JPEG, TIFF, or multiple-page PDF.