Is It Down Right Now? - Network & Admin
intoDNS - IntoDNS checks the health and configuration and provides DNS report and mail servers report.
UPDOWNtoday - UPDOWNTODAY allows you to check whether the website is down or not.
Dnsstuff - DNSstuff offers DNS tools, Network tools, Email tools, DNS reporting and IP information gathering.
IsThisSiteDown.org - Free tool to check whether a site is down just for you, or for everyone.
MxToolBox - All of your MX record, DNS, blacklist and SMTP diagnostics in one integrated tool.