PingCastle is an Active Directory auditing tool. It checks your accounts, computers and configuration in AD and gives you a great report on things that should be addressed.
Cacti is an open source network monitoring tool based on RRDTool’s data logging and graphing system. The tool uses network polling and data collection functionality to gather information on devices on networks of any size. This includes the ability to design custom scripts for data collection alongside support for SNMP polling. It then displays this information in easy-to-understand graphs which can be arranged in whatever hierarchy your business is most comfortable with.
Spiceworks Network Monitoring is a free tool offered as a part of a Spiceworks monitoring and management solution suite. Their monitoring solution offers real-time monitoring capabilities for networks and devices. You can perform ping checks to verify that your network and all connected nodes are currently functional. The tool also supports common network protocols, like HTTP and SIP, or user-defined custom protocols, as well as custom settings for alerts based on sensitivity.