mon.itor.us, a service which also provides free Web performance measurement services, appears to be having a wee problem.
The most recent GrabPERF data on this site is available here. The issue may be corrected by the time you look at the data.
I don't wish suffering like this on anyone. GrabPERF had it's own 3-4 day outage a few months ago. It's just sad to see when monitoring services go down.