Is there a convenient tool to draw a few simple network configuration diagrams?

Applications can be created using the Dia diagram drawing tool (Windows/Mac OS X/Linux).

sample diagram

I think there are many tools, so I think it's best to try them in person.
Personally, I write on Cacoo these days.It's not as good as Visio, but it's easier and easier to operate than PowerPoint.

Also, nwdiag is a tool that converts textual code into a diagram, and I sometimes use this tool.
It's a pain in the neck to draw lines one by one, so I'm happy that you can draw pictures on your own.