Open Source Tools for the Systems Administrator discusses four open source tools: Cacti, OSSEC, Nmap, and RackTables. Cacti is a frontend for storing and displaying historical monitoring data visually. OSSEC is a host-based intrusion detection system that integrates log analysis, file integrity checking, and real-time alerting. Nmap is a network discovery and security auditing tool useful for tasks like network inventory and service monitoring. RackTables is an asset management solution for documenting hardware, networks, rack layouts, and generating reports.