Visio can save documents as html files, which can be viewed by others.
Visio 2007 has PDF and XPS output capability, and lots of folks also
have 3rd party PDF exporters.
But Visio also has a free "Visio Viewer" add-on that anyone can download
to view Visio files, without having Visio installed. The viewer offers
some nice features like pan and zoom, multiple page support and access
to Shape Data (custom properties) in the shapes and document.