Port checking:
ActivePorts
TcpView
CurrPorts; IPNetInfo
Roadkil's Scan Port
TCPEye
Opened Ports Viewer
Free Port Scanner
Pwn
Port Scanner
PortScan
IPFingerPrints 
Advanced Port Scanner
SuperScan
CrowdInspect; download

Botnet diagnosis:
Trendmicro RUBotted
FayeTracker

Network traffic monitoring:
Axence NetTools
Networx
Netmeter
Network lights
Urlsnooper
Cucusoft Net Guard
ProcNetMonitor
Pandora FMS
Kismet
Network Monitor II
PRTG Network Monitor
Network Activity Indicator
NetWitness Investigator
Network Meter
My Network Speed
NetTraffic
York
Real Network Monitor
SterJo NetStalker
Active Wall Traffic Monitor
Network Manager
TechString
NetToolset

Network traffic analyzing:
Best Free Network Analysis Tools
Wireshark
NetworkMiner
Nmap
Fiddler
Cain & Abel
IP Tools
Microsoft Network Monitor
Analyzer
Smartsniff
Colasoft Capsa Free Network Analyzer
Angry IP Scanner
SoftPerfect Network Scanner
Advanced IP Scanner
WinSniff
NetworkTrafficView
RawCap
IP Traffic Spy
AthTek NetWalk Personal
Moo0 Connection Watcher
Network Scanner
ICSI Netalyzar

Whois/IP lookup:
DomainTools
DomainHostingView
WhoisThisDomain
Online Domain Tools
Whois Tool
who.is
IPNetInfo
IPInfoOffline
IP locator
ip2c

Network intrusion detection system:
Snort
Bothunter
TWIDS
ThreatFinder

Control internet traffic priority:
NetBalancer

Internet speed and quality test:
Speedtest
Pingtest
Broadband Speed Checker
JD's Auto Speed Tester
TamoSoft Throughput Test
DSL speedtest (Ch)

Ping:
Emco Ping Monitor
Ping Monitor
hrPING
http-ping
PingInfoView
Colasoft Ping Tool

Wireless tools:
Best Free Wi-Fi Network Finder Utility
inSSIDer
Xirrus Wi-Fi Inspector
NetSurveyor
SciLor's Wifi Penetrate
Wireless Network Watcher
Homedale
Network Stumbler
Wireless Wizard
TekWiFi
NetGhost
WeFi
W10ifiInfoView
Wifi Protector
SoftPerfect WiFi Guard
Securing wireless network
How To Check If Someone Is Stealing Your WiFi

Collection of tools:
Network Security Toolkit
OSSIM
Plax network suite
eToolz
Network Stuff
HoverIP
Network Monitoring Tools Package

Monitor file shares:
ShareEnum
NetShareMonitor
McAfee ShareScan; review
ShareWatcher
ShareWatch

ARP protection:
Protection against ARP poisoning
Anti NetCut
DecaffeinatID
CUTe ARP Protector

Website monitoring/analysis:
Websites Cop
Down For Everyone Or Just Me
WebESC
NotiPage
Mon.itor.us
ThreatSign!
ServiceMon
TargetAnalyser
See also in this list: Website security

Other:
NoSnoop
CountryTraceRoute
Computer IPv6 addresses & privacy
WebSiteSniffer
HTTP LogStat
DNSQuerySniffer
Connection Keeper
Sentinel
LanTopolog
Simple IP Config
ADHD
HookME
Firewall Builder
Reporting Hacked or Rooted Servers used by Booters to Datacenters
SSL Eye

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