DNS Analysis and Debugging
If you have to deal with any DNS configuration or want to do some analysis on any other existing DNS, DNSstuff.com might just do the trick for you...Including analysis such as DNS reporting (pretty useful since it can point out any errors you might have in your DNS entries), spam database lookups, DNS record retrieval, among many others, this is a time-saver and a great find. Another tool for my tool belt.
Revision: since I put up this post, DNSstuff is no longer free. A shame since this was a way cool and useful service.