Virtual machines and emulators: Oracle VirtualBox, VMware Player, VirtualPC, Parallels, and open-source options
Vulnerability scanners: OpenVAS, Metasploit
File system monitors: AIDE, Samhain, Tripwire
Windows auditing tools: Nbtstat, Cain, MBSA, PsTools
Command-line networking tools: Netcat, Cryptcat, Ncat, Socat
Port forwarders and redirectors: SSH, Datapipe, FPipe, WinRelay
Port scanners: Nmap, THC-Amap
Network sniffers and injectors: WinDump, Wireshark, ettercap, hping, kismet, aircrack, snort
Network defenses: firewalls, packet filters, and intrusion detection systems
War dialers: ToneLoc, THC-Scan, WarVOX
Web application hacking utilities: Nikto, HTTP utilities, ZAP, Sqlmap
Password cracking and brute-force tools: John the Ripper, L0phtCrack, HashCat, pwdump, THC-Hydra
Forensic utilities: dd, Sleuth Kit, Autopsy, Security Onion
Privacy tools: Ghostery, Tor, GnuPG, Truecrypt, Pidgin-OTR