But don't let the complexity discourage you—Nagios has saved my bacon more times than I can possibly recall. The early-warning systems provided by this tool for so many different aspects of the network cannot be overstated. It's easily worth the time investment. I've written several Nagios plug-ins, including one that monitors a wide variety of APC hardware, and they've paid me back many times over.

NeDi (www.nedi.ch)
If you've ever had to search for a device on your network by telnetting into switches and doing MAC address lookups, or you just wish that you could tell where a certain device is physically located (or, perhaps more important, where it was located), then you should take a good look at NeDi.

NeDi is a LAMP application that regularly walks the MAC address and ARP tables on your network switches, cataloging every device it discovers in a local database.

You can then log into the NeDi Web GUI and conduct searches to determine the switch and switch port of any device by MAC address, IP address, or DNS name.

In addition, NeDi collects as much information as possible from every network device it encounters, pulling serial numbers, firmware and software versions, current temps, module configurations, and so forth. You can even use NeDi to flag MAC addresses of devices that are missing or stolen, and NeDi will watch to see if they appear on the network again.

Configuration is straightforward, with a single config file that allows for a significant amount of customization, including the ability to skip devices based on regular expressions or network-border definitions. You can even include seed lists of devices to query if the network is separated by nondiscoverable boundaries, as in the case of an MPLS network. NeDi usually uses Cisco Discovery Protocol or Link Layer Discovery Protocol, discovering new switches and routers as it rolls through the network, then connecting to them to collect their information. Once the initial configuration has been set, running a discovery is fairly quick, and runs from cron at set intervals.

NeDi also integrates with Cacti to some degree, and if provided with the credentials to a functional Cacti installation, device discoveries will link to the associated Cacti graphs for that device.

Ntop (www.ntop.org)
Ntop is the product of a fantastically focused mind—that of Luca Deri, the project's author. Ntop is a top-notch network traffic monitor married to a fast and simple Web GUI. It's written in C and completely self-contained; you run a single process configured to watch a specific network interface, and that's about all there is to it.