In this paper we present lo2s - a lightweight performance monitoring tool to sample applications as well as the executing system. It enables the user to analyze the performance of a parallel application without requiring the time-consuming and error-prone process of application instrumentation. The collected performance data is complemented with various metric data, i.e., perf counters, kernel tracepoints, model specific registers, and custom metric data provided by plugins. Comprehensive visualization is enabled by compatibility with established tools.