I only have about (hmmm) 40 services that I'll be monitoring.
I've got about 25 defined so far.
Nagios will work.
When reviewing Zenoss, I really don't like the concept of open source crippleware / bait and switch. You know, teaser version with X capability, paid with X+.
[link|http://www.zenoss.com/product/overview#subscriptions|http://www.zenoss.co...iew#subscriptions]
And that pricing is total bullshit. They start at $66 per managed resource per year. And then say: Min 50 resources.