Free drawing software for Windows, Mac OS X, and Linux
Dia Diagram Editor is free Open Source drawing software for Windows, Mac OS X and Linux. Dia supports more than 30 different diagram types like flowcharts, network diagrams, database models. More than a thousand readymade objects help to draw professional diagrams. Dia can read and write a number of different raster and vector image formats. Software developers and database specialists can use Dia as a CASE tool to generate code skeletons from their drawings. Dia can be scripted and extended using Python.
Sodipodi is general vector illustrating application for Linux/Unix and Windows. It uses W3C SVG as native file and in-memory image format and can do many neat things.
The Doodle project aims to provide a language to easily describe Origami diagrams and produces a ready-to-print document. Doodle will free creators from diagramming constraints and increase their capability to share their creations.