Sure, Nagios, Collectd, Ganglia etc etc etc are all options. But I'm talking about something hosted, external, URL-based (so we can find user-facing problems),

Sure, Nagios, Collectd, Ganglia etc etc etc are all options. But I'm talking about something hosted, external, URL-based (so we can find user-facing problems), and most importantly simple and affordable. Something where you might be okay spending $10-$100 month rather than having to have someone manage a fine-grained internal monitoring system.