CheckDNS.net

Tests for a range of common DNS and mail server problems. Includes a test of the "postmaster" account and a server header check.

DNS Report

Wide range of DNS, nameserver and mail server tests, with step by step reports and suggested corrections for detected errors.

DNS Stuff

Large collection of diagnostic tools that can be used to perform reverse DNS lookups, ping a hostname or IP address, perform a traceroute, check the DNS cache at major ISPs and perform many additional checks.

IP2Location

Web-based tool to reverse lookup country, state, city, latitude, longitude and ISP information by IP address. Free service is limited to 20 queries per day; premium service levels available.

Sam Spade

Various diagnostic tools, including Whois and IP lookup and reverse lookup tools, and a traceroute gateway. Sam Spade is also available as a freeware package for Windows systems, which can be downloaded from the main site.

Squish DNS Checker

Free web-based service that gives you access to very sophisticated DNS reports including A, SOA, CNAME and other records, verified recursively across major internet routes.

Technical Support

Quickly find out your IP address and hostname, browser version, screen resolution and other system information.

Worth a look if you're willing to pay...