Dia is a powerful drawing tool which is ideal for creating all kinds of structured diagrams.
Such as? The program opens with all the symbols you need to create regular flowcharts. But simply choose another library from the list and you'll be able to draw logic diagrams, electrical circuits, networks, databases, simple maps and more (there are libraries for BPMN, Chemical Engineering, various Cisco devices, Gane and Sarson, REi*/ Jackson/ KAOS, SDL, Sybase, UML and more).
(Although if you don't need specific symbols then that's okay, too. Dia also includes basic tools for drawing general boxes, ellipses and polygons which you can shape to suit your needs.)
Whatever you're planning to draw, creating the diagram itself is fairly straightforward. Drag and drop a shape onto the page; resize and reposition it; then add a little text. Repeat the process with other shapes, then join them together with lines. If you've ever used a similar program then you'll be at home here very quickly.
There is also plenty of power here, though. You can group symbols together to move them as a block, for instance. Multiple alignment options help to ensure everything looks neat. Layer support can come in handy with larger diagrams, and every part of the finished chart can be customised to suit your requirements. (Choose the properties of a single box, say, and you can set its line width, style and colour, and font size, colour, alignment and more.)
And once you've finished you can export your masterpiece in a lengthy list of formats, including JPG, PNG, PS/ EPS, DXF, PDF, EMF, WMF, TEX and more.
An excellent diagram creation tool, versatile and packed with powerful features and functionality