Don’t think Visio is the only application out there to create diagrams and workflows, Dia is an excellent alternative!
With Dia you can easily create relationship diagrams and with the several object styles it has, you’re able to do some more technical work such as UML diagrams, flowcharts, and chronograms. Although the default format you can save your files will be .dia, there is an excellent export tool for several formarts including svg, png, eps, jpg, and bmp.
Best of all, it is a stable Windows and Linux program! So, you can already make this open source switch even if you are on a Windows machine.