CheckYourNetwork.net - Free Online DNS Checking and Testing Tools

iptools.com - iptools.com

    Tools to check your DNS records, A Record, MX record, PTR Record
kitterman.com - Kitterman SPF Record Checking tools
    Tools to check your DSN SPF records or Sender Policy Framework. SPF Testing
Squish.net - DNS Checker that checks the propagation of all root servers
    This is a free web-based service for DNS experts. Given a record name, and a record type, you will receive a report detailing all possible answers. This is accomplished by traversing the DNS tree from the root examining all possible routes that a client could travel, calculating percentage probabilities on the way. Note that this service does not consider your local cache, which may have information that is in conflict with the Internet DNS tree.
DNSStuff.com - DNS, Whois, Email Abuse Lookups, Email Blacklist Checking -
    This is a free web-based service for IT Professionals.
Men and Mice DigIT! - Online DIG tool
    Men and Mice. The DNS experts.
Zoneedit.com DNS Tools - Online DNS Check
    Zoneedit.com - A large DNS hosting company

Be Sure to Check out Clustered Networks Untangle Security Appliances

The Untangle Gateway, the world's first commercial-grade open source solution for blocking spam, spyware, viruses, adware and unwanted content on the network, provides a free and better alternative to costly, inflexible proprietary appliances. Untangle's applications are currently being used in tens of thousands of businesses in a variety of industries, including financial services, real estate, education, bio-science, and professional services.