I like Pingability.http://pingability.com/index.jsp
They sell monitoring services but they also have a free instant check that you can use to
check DNS, SMPT and POP3/IMAP servers.
Look for the link: "Instant Checks" right below their logo.
You can check by "dig" or "nslookup" command.
There's also http://www.dnscog.com/
which is provided by the same people at offer DynDNS.
There's takes a little longer to process but can offer more detail and information to help