Estimating the user's mental state with a set of special measuring devices can be helpful in detecting bottlenecks of the human-computer interaction. Until recent years, electroencephalography devices were too expensive and too complicated for most UX researchers, but now there are affordable consumer-grade EEG devices. The talk covers EEG headsets produced by NeuroSky and Emotiv, as well as the open hardware OpenBCI project. Each headset has its advantages and disadvantages for UI/UX research. Specifics of data that can be acquired from each headset is reviewed, and existing open-source tools and libraries to get these data are discussed. The talk explains how we can use EEG headsets to evaluate mind concentration and relaxation, as well as rejection and arousal factors, which can be used to detect positive and negative emotions. EEG usage scenarios are discussed with examples of the FLOSS projects exposed to such UI testing. Estimating the user's mental state with a set of special measuring devices can be helpful in detecting bottlenecks of the human-computer interaction. Until recent years, electroencephalography devices were too expensive and too complicated for most UX researchers, but now there are affordable consumer-grade EEG devices. The talk covers EEG headsets produced by NeuroSky and Emotiv, as well as the open hardware OpenBCI project. Each headset has its advantages and disadvantages for UI/UX research. Commercial devices have different primary goals complicating their usage for research with cyphering, special licenses and limitations in the vendor-provided proprietary SDKs - but open-source tools developed by the community improve the situation. OpenBCI is quite the opposite: it is fully open, but much harder to obtain/build. Specifics of data that can be acquired from each headset is reviewed, and existing open-source tools and libraries to get these data are discussed. The talk explains how we can use each headset to get information about the user's mental state. Mind concentration or relaxation is a highly informative parameter for UX; it can be easily evaluated and it can be obtained with any of the EEG headsets. Besides that, most of the reviewed devices provide enough data to measure rejection and arousal factors, which can be used to detect positive and negative emotions. Finally, EEG usage scenarios with examples of the FLOSS projects exposed to such UI testing are discussed.