Processor stress test utilities are important tools for a number of different use cases. In particular, cooling systems need to be tested at maximum load in order to ensure that they fulfill their specifications. Additionally, a test system characterization in terms of idle and maximum power consumption is often a prerequisite for energy efficiency research. This creates the need for a simple yet versatile tool that generates near-peak power consumption of compute nodes. While in different research areas tools such as LINPACK and Prime95 are commonly used, these tools are just highly optimized and compute intense routines that solve specific computational problems. As stress test utilities they are unnecessarily hard to use and in many cases unreliable in terms of power consumption maximization. We propose FIRESTARTER, an Open Source tool that is specifically designed to create near-peak power consumption. Our experiments show that this task cannot be achieved with generic high-level language code. We therefore use highly optimized assembly routines that take the specific properties of a given processor microarchitecture into account. A study on four compute nodes with current or last generation x86_64 processors shows that we reliably exceed the power consumption of other stress tests and create very steady power consumption patterns.