Online-Tools

Network Information Gathering

Multi purpose: clez.net, serversniff.net, Robtex, dnsstuff, tools-on.net, msv.dk, all-nettools.com, tatumweb.com, infosyssec.com, OnlienDomainTools, Heise
Whois: geektools, ripe.net (free-text), Domaintools (wildcard), arin.net
DNS-Lookup: spacereg (dig), Zone Edit, DNSstuff, NSLookup, Complete DNS Traversal, DNS Bajal (shows paths graphicly)
DNS-Special: squish.net (bind version, recursion), cuni.cz sleuth (config errors?)
Domains: Robtex (hosts/domains sharing ip/mx/ns, subdomains), serversniff (subdomains, hosts, tld), netcraft (searchdns)
Ping / Traceroute: : tracert.com (multiple), opus1.com (udp, fast), serversniff (tcp, layer4, icmp, udp), ringofsaturn (icmp, tcp), Traceroute.org (by country)
Portscan yourself: Nmap Online Scanner
E-mail: tools-on.net (open relay DB)
Routing: IP to ASN

HTTP-header: nerdlabs (version 1.0, 1.1), netcraft.com (server version, historical), serversniff
Website: Alexa Related Info, Pagerank Checker
Location: MaxMind, HostIP.info
Benchmark: serversniff (http server), wieistmeineip.de
Site performance: Akamai performance comparison
Mac-Address: coffer.com (vendor, mac)
USB devices: Endpointscan (usb, cd)
SNMP: bytesphere.com (mib download), OID repository, OID assignments

Encoding/Hashing: Translator (bin, dec, hex, base64, hash), serversniff, (string, files), ostermiller.org (url, base64, hex), TextOpus
File viewing: PDF, PS, DOC viewer, winmail.dat
File conversion: cometdocs (file format guesser, pdf2txt, xls2csv, dos2unix), Zamzar, vCard to LDIF/CSV
Video tools: filsh
Unit Conversion: OnlineConversion.com
Other: AT&T Text to speech

HTTP2XY-Gateways: SurFTP (ftp), mosha.net (telnet, registration needed), Page2RSS
OneTime-Email: Mailinator, SpamGourmet

Online office: Zoho
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.