Visio is a simple tool widely used by IT professionals to draw flow charts and data flow diagrams .
Microsoft has released a free Visio viewer, which allows users to open and view Visio diagrams.The free Visio viewer is ideal for users who had to just open and use the Visio diagrams as reference.
If visio viewer is installed on your pc, simply double-click a Visio drawing i.e. files with .vsd, .vss, .vst, .vdx, .vsx, or .vtx extension. The Visio Viewer, loads and renders the drawing in the IE 5/6/7/8 browser. If IE is already open, you can simply drag and drop the visio diagram to view it.
Download from here