A list of utilities and websites that I have found useful in system and network administration.
Software
TCPing: Ping a TCP port
Why it’s useful: The default Ping application in Windows will tell you that a device is responding on the network or Internet. TCPing takes this a stage further, and will let you know that a particular TCP port on the destination device is responding.
Websites
MXToolbox: Check MX records, analyze mail headers, blacklist checks.
Unlock the Inbox: Email testing.
DNS Query: Whois, DNS, IP lookups.