Visual Network Topology Map?
Solution 1:
The latest nmap comes with a flashy GUI that can graph network topology.
Solution 2:
We use NetworkView which works quite well.
Solution 3:
Have you tried something like Spiceworks? You tell it the network to scan and it will identify nearly any device on the network. Feed it SNMP and login information and it will pull specific information from those devices to help map the network. They have a network mapping utility that is still in beta, but it seems to work decently well.
Best of all, it's free.