Dia is free and open source general-purpose diagramming software, roughly inspired by Microsoft Visio. Dia uses a controlled single document interface (SDI) similar to GIMP and Inkscape.
Dia has a modular design with several shape packages available for different needs: flowchart, network diagrams, circuit diagrams, and more. It does not restrict symbols and connectors from various categories from being placed together.
Dia has special objects to help draw entity-relationship models (obsoleted tedia2sql or newer parsediasql can be used to create the SQL DDL), Unified Modeling Language (UML) diagrams, flowcharts, network diagrams, and simple electrical circuits. It is also possible to add support for new shapes by writing simple XML files, using a subset of Scalable Vector Graphics (SVG) to draw the shape.
Dia loads and saves diagrams in a custom XML format which is, by default, gzipped to save space. It can print large diagrams spanning multiple pages and can also be scripted using the Python programming language.
Dia is available for Windows, Mac, and Linux and is available to download at no cost here.