Open Source Network Managment Software?

A loner from the Anonymous Coward Network asks: "I do systems administration for a company with a relatively large and complex all Cisco network. Our company has grown tremendously in a very short period of time and the network has had to grow with it. The collection of simple tools just don't cut it anymore and I've been asked to research commercial network management systems. Most of the ones we've evaluated so far have been very expensive, and none of them properly handle all aspects of our network. Being closed source and proprietary there is nothing you could do with any of these but live with whats been provided, or hope the vendor would make the needed changes, (and hope we can afford the cost of them doing so). Are there any good Open Source network management packages out there? In particular, is there anything written that takes layer 2 switching into consideration, or gathers and correlates spanning tree information?"