The following is a quick reference list of tools found helpful by 3Essentials administrators:

DNS

• DNS query/lookup
  • http://dnsquery.org the DNS RECORD QUERY does a really nice job in its results of displaying and describing the recursive query process from root servers, to TLD nameservers, and then on to authoritative nameservers. 
• DNS tree: determine which root servers are authoritative for a given TLD.
  • http://dnstree.com/
• DNS traversal: make sure all ROOT nameservers are indicating the same list of authoritative nameservers for your domain.
  • http://www.dnsqueries.com/en/dns_traversal.php
  • http://www.squish.net/dnscheck/
  • http://dnsquery.org/ has a DNS traversal check as well
• DNS Propagation: perform a DNS lookup against a selection of random nameservers around the world.  Especially useful to check the current state of DNS propagation after making changes to your domains:
  • http://www.whatsmydns.net/#A
• PTR(rDNS) lookup: a PTR record is required for an IP address that relays mail.
  • http://postmaster.aol.com/cgi-bin/plugh/rdns.pl
• DNS check/test (including glue records) evaluating your DNS health
  • http://www.webdnstools.com/dnstools/domain_check

Domain Registration

• WHOIS: see who a domain is registered through, who is the domain registrar, when it expires, and what nameservers are authoritative (generally, the hosting company).  These particular WHOIS tools do one thing that most don't... they give you both the REGISTRAR and the REGISTERED THROUGH (which is the domain registration reseller, if the domain was registered through a reseller).
  • http://centralops.net/co/DomainDossier.aspx
  • http://who.is/ (scroll down for the registered through info)
  • http://betterwhois.com/ scroll down for all info, but this site is also query rate limited)
  • http://technet.microsoft.com/en-us/sysinternals/bb897435 A WHOIS command line for domain registration info from Mark Russinovich of Sysinternals 

IP Addresses/Networks

• ARIN, who owns a given IP / network ranges
  • https://www.arin.net/
• geographic information about IP / network ranges
  • http://www.maxmind.com
  • http://geoip.flagfox.net
• Check Network Ports
  • http://www.mynetworktest.com/ports.php
  • http://www.ipfingerprints.com/portscan.php
• Global undersea telecommunications cables
  • http://www.cablemap.info/

Security

• Virustotal is a service that analyzes suspicious files and URLs and vacilitates detection of malware:
  • http://www.virustotal.com/
• Point unmaskparasites.com to a page on your site and it will show you all links from your site code... revealing if a hacker has inserted hidden links inside your code.
  • http://unmaskparasites.com
• Sucuri's Site Check: A great tool that scans a site (multiple URLs) looking for malware, malicious javascript, malicious iframes, drive-by downloads, suspicious redirections, and more.
  • http://sitecheck.sucuri.net/scanner/
• University of California Santa Barbara's Wepawet, although not updated since 2010 apparently.
  • http://wepawet.iseclab.org/index.php
• AVG Online Web Page Scanner
  • http://www.avg.com.au/resources/web-page-scanner/
• BitDefender Traffic Light
  • http://trafficlight.bitdefender.com/info?url=
• McAfee SiteAdvisor
  • http://www.siteadvisor.com/ use the View A Site Report function middle right side of page. 

Encoding/Decoding tools

• http://meyerweb.com/eric/tools/dencoder/
• http://www.opinionatedgeek.com/dotnet/tools/base64decode/
• http://www.blooberry.com/indexdot/html/topics/urlencoding.htm

Misc

• Misc - Unlock The Inbox is a swiss army knife of tools... open relay tester, IP blacklist/whitelist tester, DNS record queries, WHOIS, port scanning/testing, SPF/DK/DKIM testing, IP4 to IP6 conversion, IP range to CIDR conversion, IP geo location, etc
  • http://unlocktheinbox.com/
• TELNET - web based telnet utility (for ssh and telnet)
  • http://sshterm.com/forum/

FileManagement

• create a file of a specific size (for testing purposes)
  • fsutil file create new (filename) (filesize)
  • example: fsutil file createnew c:\testfile.txt 10480000