JSysmon is a Java library providing access to system information like CPU or Memory usage on various platforms using native calls. It can be used to build cross-platform monitors in your applications.
inspectorJ is a cross platform java profiler with a graphical client that allows for profiling remote jvms. inspectorj is pre-alpha and is currently under development.
A simple, cross platform performance monitoring application specifically designed to be used with nVidia's instrumented driver and the NVPerfSDK to give a graphical representation of internal GPU counters. Support for non-GPU counters is also available.