Dia
is free and open source

Free and open source software

Free and open-source software or free/libre/open-source software is software that is liberally licensed to grant users the right to use, study, change, and improve its design through the availability of its source code...

 general-purpose diagramming software

Diagramming software

Diagramming software is a computer program that is used to produce graphical diagrams. Diagrams are used in software and business to represent dataflows, workflows and software architecture.- Flowchart :* ARIS Express * Creately...

, developed originally by Alexander Larsson. Dia uses a controlled single document interface

Single document interface

In graphical user interfaces, a single document interface or SDI is a method of organizing graphical user interface applications into individual windows that the operating system's window manager handles separately. Each window contains its own menu or tool bar, and does not have a "background"...

 (CSDI) similar to GIMP

GIMP

GIMP is a free software raster graphics editor. It is primarily employed as an image retouching and editing tool and is freely available in versions tailored for most popular operating systems including Microsoft Windows, Apple Mac OS X, and Linux.In addition to detailed image retouching and...

 and Sodipodi

Sodipodi

Sodipodi is an open source vector graphics editor, discontinued in 2004, which is the predecessor to Inkscape.-Development:Sodipodi started as a fork of Gill, a vector-graphics program written by Raph Levien. The main author is Lauris Kaplinski, and several other people have contributed to the...

.
Dia has a modular design with several shape packages available for different needs: flowchart

Flowchart

A flowchart is a type of diagram that represents an algorithm or process, showing the steps as boxes of various kinds, and their order by connecting these with arrows. This diagrammatic representation can give a step-by-step solution to a given problem. Process operations are represented in these...

, network diagram

Network diagram

A computer network diagram is a schematic depicting the nodes and connections amongst nodes in a computer network or, more generally, any telecommunications network.-Symbolization:...

s, circuit diagram

Circuit diagram

A circuit diagram is a simplified conventional graphical representation of an electrical circuit...

s, 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 model

Entity-relationship model

In software engineering, an entity-relationship model is an abstract and conceptual representation of data. Entity-relationship modeling is a database modeling method, used to produce a type of conceptual schema or semantic data model of a system, often a relational database, and its requirements...

s (tedia2sql can be used to create the SQL DDL), Unified Modeling Language

Unified Modeling Language

Unified Modeling Language is a standardized general-purpose modeling language in the field of object-oriented software engineering. The standard is managed, and was created, by the Object Management Group...

 (UML) diagrams, flowchart

Flowchart

A flowchart is a type of diagram that represents an algorithm or process, showing the steps as boxes of various kinds, and their order by connecting these with arrows. This diagrammatic representation can give a step-by-step solution to a given problem. Process operations are represented in these...

s, network diagrams, and simple electrical circuits.