[bobfunk]

We use Nagios combined with Ganglia for webpop.com as well and it's a great solution.

But if we weren't on a tight budget (we're bootstrapped) I would have gone with something like CloudKick no doubt.

The server for Nagios might be cheap, but installing, refining and updating our monitors does take time that could be used on developing our product instead.

[BCGC]

I think it depends on how many servers you monitor :)

$99 per month for 6 servers is really cool.