Nmap

I have been using Nmap for a long time now. It is a really good command line network scanning tool. I use it for almost all my scanning needs. There is also a graphical frontend available, Zenmap. It has a simple interface with some pre defined commands and provision for entering commands manually. But the feature I like most is its ability to draw topologies. Run traceroute for different IPs and it will draw a Network Topology for you.