Is anyone using a free network and server monitoring tool, like Nagios,
Zenoss, Angel, Big Sister, etc, that they like?  how easy was it to set up?

I'd like to have a console running near my desk that would tell me at a
minimum if all the routers, switches, and wireless access points are up,
our internet connection is OK, and on servers, know that httpd is in fact
running on web servers, and there's plenty of free disk space on the file
servers.