Perhaps the last area of Microsoft Office that didn’t have a direct competitor is Visio. The nearest competitors are probably Libre Office’s Draw component, and dia. Highly capable as a flowcharting and diagramming tool in its own right, dia can load and save Visio diagrams and can thus be considered a competitor to Visio. (Although it can only use the open and documented .vdx format, rather than Visio’s default .vsd)
Unlike Visio, however, dia is designed more for informal graphs, and is therefore probably better suited for the home or student user than a more restrictive package such as Visio. In our deeply unscientific teseting, dia proved fast, and capable at entity-relationship diagrams, flowcharts or simple diagramming. Most of the standard functions you’d expect to find in a diagramming package are here – connectors between objects, aligning objects relative to each other…. the list goes on and on (should you be interested, you can find some excellent sources of documentation on the dia website, incidentally…)
So, next time you need to create a professional looking diagram, draw a floor plan, or just diagram out something you need (I used used it to flowchart a short story I was writing), why not give dia a try before shelling out for Visio?