Managing Huge Networks with Open Source Tools?

An anonymous reader asks: "I work for a large multinational firm with a network that spans the globe and am responsible for evaluating the software we use to monitor our network. Our department has a lot of money, and we're usually willing and able to spend it on good commercial software. Recently though, I find myself evaluating and approving more and more open source software. We are actually in the process of replacing some of our commercial tools with software like Nagios, LooperNG and syslog-ng. We are also evaluating MRTG, RRDTool, ntop and a host of other tools. The problem is that there's just too many of them, most of which are not maintained anymore. Here's my question: What other open-source tools do you use to monitor your networks? I not just looking for names, but how long you've been using them for, how easy / hard is it to administer and I guess how well it scales as the network grows. More importantly, are their respective projects still alive and kicking?"