This is a bit of shameless self-advertising - I figured that if I don't advertise a little bit, I never know if there are some people that might benefit of this thing.
Basically, if you want to scratch a network with a couple of routers and a cloud, now you do not need to start visio, dia, or anything else for that matter: just go to http://employees.org/~andin/network/, draw the network, and snap the picture from the screen.
Of course, it's all web2.x enabled and jazz, so you can save the layout as JSON, and load it later. Even, in a demonstration of a blatant violation of the security, you can pull in the JSON from another domain.
A little bit of history: as I use linux at work pretty much exclusively, I was always curious to have something simple to draw the diagrams. Dia is an excellent program, but I wanted something even simpler.
And so when I saw the Draw2D library I thought "ah-ha!".
The editor is not way too much more than a combination of the existing components - you can see the ingredients on its page.
Well, enough of me talking, go try it and let me know :-)