Check a domain name
Yet another one
Check a domain name health for NS and MX
another one
another one
ICANN Whois whois server
Check domain outside USA (at RIPE)
Check ip at ARIN server
a huge source of traceroute and more utils
Huge traceroute sites list
RIPE net looking glass Allow you to do traceroute and AS as well
A good Util using Dig perform domain check
Another good tool
Port scanner page
Subnet Mask util
RIPE provide subnet-mask table
UnDotted IP to Regular IP converter
IP Address explain (good One)

Wireshark is a free Network Monitor
Wireless sniffer by UFASoft is also very good one

Old version of lot of well known softwares can be found here
Look for a non existed site ? try your luck here (with wayback machine internet arciver)