Open source network topology mapper?

Greetings fellow sysadmins!

I've been tasked with creating a full map of our entire network toplology (routers/switches/vpn concentrators/firewalls, the works) by my evil overlords, and given a budget of exactly zero dollars to do it.

Can anyone recommend a good open source topology mapper, similar to the one SolarWinds makes but without the SolarWinds price tag?