We use Wormly for external monitoring. The one thing it has going over Pingdom is that they do outbound calls too, not just texts. It's nice to have that layer of redundancy (on top of PagerDuty + Nagios for internal monitoring).
My company has tried quite a few, but in the end settled on Panopta. No false alerts which many of the others suffered from. Very helpful support staff. And pretty in-depth and complete feature set.
We use Pingdom for some things but the UI is a bit awkward and drill-down analytics doesn't seem possible.
New Relic (powerful) or Site Uptime (basic and cheap)