I can put in a plug for Ganglia - I just set it up
at the end of December and was pleasantly surprised at how easy it was
using the EPEL repository packages for CentOS/Red Hat. I had it up and
running on the central manager with just a couple of lines of config tweaks,
including starting the condor_gangliad. The fact that it aggregates data
from clusters of machines right out of the box was a big benefit for what
I'm working on now - I needed something that could give me a comprehensive
overview of the pool quickly without a lot of tinkering, and that's just
what I got.