yEd

yEd is a unique tool that lets you create high-quality diagrams in the most straight forward and simple way possible. You can either create your own diagrams from scratch, hand-drawing them with the mouse, or import an Excel file, which you can then edit and modify however you like. Also, once you have finished a diagram you can export it in any format you like such as PNG, JPG, SVG, PDF, SWF,...