Network Monitoring Tools For Unix?

drexle asks: "I work for a city government supporting the various WAN's and LAN's used throughout the city by the police, fire department, etc. Currently, we are using an application on a HP-UX system on its last legs to monitor the status of the various routers, switches, etc., throughout the city, and basically just pages someone if something goes really wrong. Are there any good tools to do this available for a Linux/*BSD platform? Preferably with some sort of GUI which can display an organizational map of the network? Most of the routers/switches are from Cisco, w/ various other equipment used for the WAN connections(microwave, T1's, ...) "