I've mentioned before on here that I'm using Pingdom, and am quite happy with it. For the price it's awesome.
One of the features that took us to them is that they have monitoring sites all over the world. Our hope was that this would give us a cheap way to tell if something in our routing is b0rked, and some part of the world can't see us. Unfortunately, they'll only alert if two different sites can't see you.
What I'm looking for is a similar monitoring system that will tell me if any individual site can't get to me. Some logic on their side to tell the difference between me being out and them being out would be great, but I'll take it even without.
[Edit] Some clarification: I've only got one site (both logically and physically) that I want to monitor from many places.
[Edit 2] I'm happy to pay for this service. I'm already paying Pingdom, and probably would continue to do so even with this new service, assuming that they don't have a huge overlap in what they do.