This past days I need the Visio program from Microsoft but the price is a little bit over my budget so I tried to find some alternatives and here are the results.

ArgoUML

Available for: Windows Mac Linux Unix Java
ArgoUML is a great UML (Unified Modeling Language) tool. Written in Java and using Java Web Start makes it easy to work with (install) and use on any platform. It has full support for the UML 1.4 standard and provides excellent features to drawing and manipulating you UML diagrams. Like many other UML tools of this caliber ArgoUML provides code generation. The following programming languages are support for code generation: Java, C++, C# and PHP. The Java support is best since it can be used for round-trip engineering using the reverse engineering feature. ArgoUML also has partial support for user models such as decision model, goals model, etc. Finally diagrams can be exported in many graphical formats, primarily GIF, PNG, PS and SVG.

StarUML
Available for: Windows

StarUML is a great open source UML application. Supporting UML 2.0 and MDA (Model Driven Architecture) StarUML let’s you work with all related diagrams. Code generation can be done for Java, C++ and C# - and documentation can be generated for all of the Microsoft Office suite programs such as Word, Excel and PowerPoint. StarUML is easily extensible through modules using a COM-based plug-in architecture - and by use of templates allowing you a make new code generators. StarUML even imports from Rational Rose.

Dia
Available for: Windows Linux Unix
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, flow charts, 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.

It can load and save diagrams to a custom XML format (gzipped by default, to save space), can export diagrams to EPS or SVG formats and can print diagrams (including ones that span multiple pages).