Free uptime checking sites / services?

Unfortunately, I am unaware of an reliable services that provide more information on uptime. Pingdom is a free service, but can be unreliable at times as all it does in ping the server and does not give any further information such as a particular service being down. Pinging the server like they offer also can report false positives as well because any issue between them and us, such as an IP being blocked for too many requests from them would show the server down when it is not. There are monitoring tools out there that would reliably report uptime, but I am unaware of any free tools.