[Skunkworks] Network Diagram Images

Guys, I'm trying to come up with a network Diagram. I don't have MS. Visio seeing as im running Ubuntu anyway. Is there any other similar software i can run that will assist me come up with a network Diagram? Alternatively is there a site I can download the individual images e.g. firewall, router, PC without downloading custom made ones then editing. Regards, Eric Mugo.