Another vote for wasitup.com. I use wasitup.com, host-tracker.com, and basicstate.com, and wasitup.com is consistently the first one to report an outage, follow

Another vote for wasitup.com. I use wasitup.com, host-tracker.com, and basicstate.com, and wasitup.com is consistently the first one to report an outage, followed by basicstate, which has a surprisingly gloomy and odd interface, yet good service, and then followed by host-tracker.com, which sucks pretty much on all levels. All are free btw.