A neat and handy SSL Site Check tool. It not only checks presence of SSL certificates for your web site but also helps detect quality of SSL connection to your web site.
It can be also used to test deployment of SSL certificates on your website (web server). It extracts good amount of useful information. Good for IT pros and software test engineers.
Another useful tool is www.dnsstuff.com. It helps network administrators (and IT pros) to diagnose problems with their DNS servers.
www.dnsstuff.com has few more tools in their labs pages such as Vector Trace, DNS Traversal and Speed test etc…
Another useful site is www.network-tools.com. It provides bunch of online tools such as Ping, Lookup, Trace, Whois (IDN Conversion Tool), DNS Records (Advanced Tool), Network Lookup, Spam Blacklist Check, Convert Base-10 to IP , URL Decode, URL Encode, HTTP Headers, Email Verification
If you know any other free online tools related to networking then let everyone know in comments below.