Here are some of the best tools for testing your DNS and Reverse DNS. These tools can help you solve any DNS issues that your server may have. If you cannot solve your problems try contacting your ISP's DNS admin.
Links in bold are highly recommended tools.
DNSDig is a web version of dig. Allows you to query for a domain's DNS from any machine on the internet. Has very comprehensive results.
Online utility for DNS administrators. Test your DNS servers, domain registration, mail and web servers.
Check the delegation of your domain. Has interesting graphical results.
Provides a free DNS report on your domain. Includes over 20 different tests, looking for both minor and major problems.
DNS Search Gateway
Handy web-based DNS lookup service, with links.
Many free web-based DNS tools all in one location. Includes standard DNS lookups, WHOIS/IPWHOIS, and even obscure IPv6, IP-to-country and spam database lookups. Fast and thorough.
DNS, Whois, Nslookup, Head HTTP
Domain Name System Utility, online web interface to query DNS, Whois, Nslookup and Domain Names by IP address, Host address, Domain Name, URL.
eLiteral - Web Utility
Provides ping, traceroute, whois, finger, nslookup, and dig.
Web-based nslookup tool that lets you look up IP address information. Query their and other DNS domain name servers to look up and find IP address information of computers on the internet.
najk.com offers some ping, tracert, nslookup, and basic WHOIS for web users.
squish.net DNS Checker
Complete DNS traversal check. Shows you every DNS server that is (or may be) used for a DNS lookup, very comprehensive.
Web-Based NSlookup page
You can query any domain here to find the DNS information similar to a nslookup. Links to whois information.
Free online DNS Dig tool, similar to the UNIX "dig" command.