Recommend me an external site monitor

[Dieter]

I'm looking for an external monitoring service that can monitor websites (HTTP & HTTPS), page loads, SMTP response, DNS response, etc. We'd like notifications via email or SMS.

There seem to be quite a few companies in this game, but I have no way of knowing who's trustworthy and actually does a good job.

I've come across monitor.us, neustar.biz, securityspace.com, and quite a few others, and it's hard to tell who's legit.

Who do you use/recommend? This will be for business use so I'm willing to pay for it.

Thanks!