I hope this thread complements rather than hijacks eternal_noob's recent thread "What's the software you can't live without?"
- whois.domaintools.com - I've been using this Whois service for well over 10 years. Is there a better one? Note: There is a limit to the number of monthly lookups for non-paid users.
- viewdns.info - Easy-to-use DNS tools.
- camelcamelcamel.com - Amazon price tracker. Find out if you're really getting a good deal. I use it by entering the product's ASIN or ISBN in the search.