(now often found at diagrams.net) is the industry leader for free diagramming. It is open-source, runs in your browser (or as a desktop app), and can open and edit .vsd files (with some limitations). It supports all the shapes you need for flowcharts, UML, and network diagrams. It is completely free, with no trial limits.
Since Microsoft's servers no longer distribute the file, users must rely on physical backup media (the original CD-ROM) or verified digital archives that preserve original, unaltered ISO files. Where to Find a Safe Download File
Part of the free LibreOffice suite, is a vector graphics editor that can export to PDF and import some Visio files. While it lacks Visio's advanced stencils, it is more than capable for home and small business diagrams.
: You can sign up for a one-month free trial of the latest Visio Plan 2 to use the modern desktop and web apps. Security Warning
If you already own a legitimate license for Visio 2007 and are looking to install it, ensure you are using your original installation media and key.
