Online-Tools

Network Information Gathering

Multi purpose: clez.net, serversniff.net, Robtex, dnsstuff (testbed), tools-on.net, msv.dk, all-nettools.com, nerdlabs, tatumweb.com, infosyssec.com, Heise
Whois: CompleteWhois, geektools, ripe.net (free-text), Domaintools (wildcard), arin.net
DNS-Lookup: spacereg (dig), Zone Edit, DNSstuff, NSLookup, Domaindsdb.net, Dig it, Complete DNS Traversal, DNS Bajal (shows paths graphicly)
DNS-Special: squish.net (bind version, recursion), dnsreport (config errors?), cuni.cz sleuth (config errors?), tomDNS
Domains: Robtex (hosts/domains sharing ip/mx/ns, subdomains), searchmee.com (domain by ip), serversniff (subdomains, hosts, tld), netcraft (searchdns), Domain Converter
Ping: tools.rootmode (icmp, tcp, http) tracert.com (multiple)
Traceroute: opus1.com (udp, fast), serversniff (tcp, layer4, icmp, udp), ringofsaturn (icmp, tcp), Traceroute.org (by country), Visual Route (java, German)
Portscan yourself: nmap-online
E-mail: dnsreport (test mail), zoneedit.com (from, to; smtp banner), zoneedit.com (pop3), tools.rootmode (real time blacklist), tools-on.net (open relay DB)
Routing: IP to ASN

Anti-Virus (file): virustotal.com (upload), Kaspersky (upload), Norman (upload, mailreply), drweb.com, Trend Micro (java), serversniff
Anti-Virus (mail): gfi.com (client vulns, eicar), heise.de (client vulns), declude.com (eicar by mail)
File Analysis: ASN decoder, TCPdump
Java File Management: DiffAnywhere, Base64Decoder, JHashCalc, JReplaceThem
XML: XPath Expression Testbed, SoapTools

HTTP2XY-Gateways: SurFTP (ftp), wachtelnet (ftp), mosha.net (telnet, registration needed), Meebo, (instant messaging), Page2RSS
Java Network Clients: Mindterm (ssh, java) J-FTP (ftp), Pooka (email), Columba (email), ddnscl (dyndns), iSQL (sql), Popplet (pop3), SSHTerm (ssh), LDAP Browser/Editor
Java Network Server: Bremsserver (http), JAP (anonymity)
OneTime-Email: Mailinator, SpamGourmet

Online office: Zoho, EyeOS
Time: Local time
Calendar: mobrien.com, WebUtil Calendar, Worldtime Converter
Telephone: Billiger-telefonieren (least cost routing, DE), Das Örtliche (inverse lookup, Germany), GelbeSeiten.de (yellow pages DE), Teldir.com (international)
Maps/Routing: Google Maps, Map24 (routing, EU), stadtplandienst.de (city maps DE)
Translation: dict.leo.org, Google, Babel Fish, all-nettools.com
Other: Typetester (test fonts online)

General: Google.com, AllTheWeb.com (by language, date), Teoma (by language, region, date), MSN Search, Yahoo
Meta: Mamma.com, Dogpile, Metacrawler, Clusty, Ixquick, iZito, KartOO (needs Flash)
Security: Scurn (mailing lists)
Science: Citeseer (computer science), Google Scholar
Mobile: Wikipedia (PDA)
Other: Pagefactor (Google cache, Wayback), Wayback machine (historical WWW), Searchlores.org (about searching), Langenberg.com (phone, reference, geographic, ...), Yahoo vs. Google (compare results)

You might also be interested in my Quick References section.