DNS seems to be one of those areas that lots of people get completely lost and confused on for some reason.
Fortunately DNS is one of the things I like.  So here in the coming Millennia I will stuff some nice info / links for you and hopefully explain some of the common and confusing DNS issues.  Though most of this will be angled around the Cobalt/Sun Raq3's and Raq4's.  But it will also cover the general stuff on DNS as well.

DNS Resolver Status Checking - Perl Script to verify *any* Resolver is UP and responding.

• DNS Resolver Status - This is mainly aimed for my UK2 RaQ friends who have terrible trouble with UK2's Resolvers that frequently disappear without reason or explanation and cause immense problems from e-mail to ssh logins.
• DNS Resolver Status - Source Code - Download the Source code and / or Binaries and run from Your PC/Server.

DNS/NameServer/Domain Lookups

• Universal WHOIS - WHOIS Service, that's able to search all of the current official WHOIS servers
• NSI NameServer Lookup - THE place to go to check if your IP has a Name Server Host Record attached to it. But thanks to ICANN's insanity on Jan 2003 they split the .org domains away from its old friends. This now means if you ever want to setup a NameServer Host Record (Glue Record), you have to mess about with mutiple addions, you can see the .org end results here: (example of one of our NameServers) http://www.pir.org/. If you have access to a OpenSRS account, you can have a NameServer added as Host Records (Glue Records) to all the major Registras in one go... sadly this is NOT an option currently for end-users, only the owners of the SRS account. In my view this should be a STANDARD feature that happens every time you add a NSHR... but thats common sense, so that wont happen!
• NameServer Lookup - Joker (tad confusing and disorganized)
• Demon - Demons Lookup Services
• Uniplace CheckDNS - A useful tool to lookup and explain your DNS in simple terms
• Whois Source - Domain name search tool that allows a wildcard search of all current / deleted / expired whois domains. - Listed by Steve Coe, Thanks

NOT Free Secondary DNS Services

• CamelDNS - A commercial service that im a partner of. CamelDNS provides both Primary & Secondary DNS services. This service is squarly aimed at those that need DNS for mutiple domains, and that think running DNS on the same machine as the websites is not the wisest of moves. We offer DNS servers that are deliberatly spread throughout the world, on different continents, to provide extreemly high availability, and keep the speed of DNS queries as high as possible for the visitors from those continents.
  This service links (optionally) with our 'fallover' MX (mail) services.

Free Secondary DNS Services (UK2 Raq3 & Raq4 owners only)

• IdentifiedServer - Nathan's Free Secondary DNS Hosting (UK) - My Web Interface :)
• Max Hosting - Chris B's Free Secondary DNS Hosting (USA)