I want to create an "architecture diagram" to explain our Software (libraries, Eclipse RCP, modules, features, etc.).
You can use whatever program you want as long as it can draw colored shapes.
Some people I know use PowerPoint, since you can make nice designs with it.
Or in Visio: New->General->Block Diagram. You can alter the design by right click->format->line. (You can make it more beautiful but I didn't want to spend to much time)