Pingdom GIGRIB is a free, unique, distributed uptime monitoring service. GIGRIB users can add websites they want monitored in return for letting their own computers become a part of the GIGRIB monitoring network.
The uptime information for the thousands of websites that have been added by GIGRIB users is public and searchable. Each GIGRIB user can add up to 10 different websites for monitoring.
Pingdom GIGRIB - details 
