Spiceworks; Hardware/Software Inventory. Required for many certifications.
AlienVault; EUBA End User Behavioral Analysis. Detect odd behavior of malware/insider threats.
OpenVAS; Vulnerability Scanner. Scan workstations & servers for security problems. No client required.
NMAP; fast subnet scanner, requires WinPcap or NpCap
Angry IP scanner; minimal information but portable and doesn’t require special drivers.
PRTG Monitor; free for up to 100 devices. Nice auto scan for services. no client required.
NirSoft NetBScanner; scan your subnet for NetBios. NetBios should be turned off in a Domain environment.
MailSniper is a tool to test eMail passwords.