Dynagen
Dynagen is a front end for Dynamips, a Cisco router simulator. It is very popular within educational institutions.
Dynagen can be used to, very easily, construct a virtual network of routers, switches, hubs and hosts. These virtual devices can then even be connected through the physical interface of the Dynamips server, to physical hardware.
Dynagen can be used with GNS3 - a graphical front-end.