Dia is designed to be much like the commercial Windows program 'Visio'. It can be used to draw many different kinds of diagrams. It currently has special objects to help draw entity relationship diagrams, UML diagrams, flowcharts, network diagrams, and simple circuits. It is also possible to add support for new shapes by writing simple XML files, using a subset of SVG to draw the shape. I recommend you get a fairly recent version, but not version 1.8.0.
Binaries are available for some platforms.
If you have a recent Linux installation, you may have all the pieces already. After clicking OK on the preferences, you can make a diagram. They resize for the paper size. However, we'll come back to this later. After clicking OK, the page setup goes away and you are ready to populate the diagram.