I was unaware of this project, but thanks to stephenpc on twitter, I read an excellent (if a bit dated) article on RootDev which brought OpenNMS to my attention as a possible replacement for nagios.
I was mildly surprised, mostly because I haven't been shopping for a Nagios replacement since I installed and configured Nagios 3. I had looked at Zenoss as a possibility, but decided to stick with Nagios since I was already familiar with the configuration routine (and 3.x had some great improvements in that regard).
Judging by Craig's comments in that article, openNMS solves problems that I don't have, like so many hosts or services that you can't get to the bottom of the summary page without it refreshing. That's a problem I'm glad I don't have, but I know that some of you run very large networks. So my question would be, what do you large-network guys use for monitoring? And if you have a small network, do you use monitoring? I remember back to the time before I knew about monitoring solutions like Nagios, and it scares me to death. I would actually manually check on important services to make sure they were up, and that was my only way of doing it.
Of course, in my defense I was young and naive then, and didn't even backup things to tape. Ah, the folly of youth.