CentOS (short for Community ENTerprise Operating System) is a free, open-source, enterprise-class and community-supported computing platform that derived from the same source code with Red Hat Enterprise Linux OS. At present, there have been increasing Internet services or applications around the world that require high stability (including a variety of purposes of servers and business applications) are running on it, rather than RHEL.
The god knows who is using L0phtCrack as a security auditing tool to do the so-called password strength testing or recovery. I conclude that 90% of individuals are using it to violently obtain the passwords itself (through methods of dictionary, brute-force, hybrid attacks, rainbow tables, etc.). Whatever you's like to use it for, what we just can't deny is that it's the most outstanding one in similar software.
As a network or website administrator, most often we need know everything quite well about the LAN or even WAN our hosts lying in. For example: the name of each computer in local network, network card address, IP address, shared resource, open ports; look up WHOIS information such as domain name, host provider, DNS... about some website in detail. For these, IP-Tools can become a good assistant for our network/Internet management.
For guy who wants to be a master of network (WAN/LAN, no matter it's black, red or white), without help of various off-the-shelf classical network utilities for multipurpose uses, I'm afraid your level will stay in the DOS era of last century forever. I mean, in that case, even it was you invented the whole Internet, you could not do anything interesting or exciting.
Sysinternals Suite is a tool set originally developed by Winternals company for internal use, aimed at resolving all sorts of problems that engineers might encounter in their usual work. Later it was placed on the Internet for free downloads, and some tools also opened source. Till today, Sysinternals has long been popular with high praise in the IT industry.