Diagram Designer (free)
is another freeware tool that looks a bit outdated but the ease of use should make it appealing to a wide variety of users. Unlike Dia, Diagram Designer has a much larger selection of symbols and icons. The only thing I didn't much care for with Diagram Designer is that the connector tool is a free-form tool so you actually have to manually draw your connections between machines. Outside of that little drawback, DD is a solid entry.