With all other mentioned caveats (geographic dispersion, intermediate network latency, time of day in relation to traffic etc), there are a number of specialist websites which solve this problem:
http://host-tracker.com - returns Location, Result, Page Size, Response time, KB/sec, IP, Partner
http://www.dotcom-monitor.com - returns Monitoring Location, Monitoring Date and Time, Duration(msec), Status, Error Description, Error Number, Reason
I find host-tracker the most comprehensive. Of course, you will need to know some hosts on the providers you are testing to use this method. Alternatively (assuming they're eating their own dogfood and host themselves) you could try testing their homepages, although it's likely they'll be prioritising traffic to their own domain.
edit: and another - http://wheresitup.com/ - very simple but effective interface