SIGAR (System Information Gatherer and Reporter) is a cross-platform, cross-language library and command-line tool for accessing operating system and hardware level information in Java, Perl and .NET.
Hyperic is application monitoring and performance management for virtual, physical, and cloud infrastructures. Auto-discover resources of 75+ technologies, including vSphere, and collect availability, performance, utilization, and throughput metrics.
System Information Monitor is a java based program that monitors various system information, it supports many platform including Windows, Linux and Unix. The underlying API utilizes Hyperic SIGAR API, JFreeChart and Swing Application Framework.