Frontier Nerds have been experimenting with brain wave tech as part of their Mental Block project.
In this well documented project they take the headset from Mattel’s Mind Flex game and hack it to communicate with an Arduino board to measure brain waves and display their levels graphically on a PC via Processing. They chose the Mind Flex device because the board gives access to the FFT of the waves and the relatively low hardware cost.
In addition to the hardware, you need the Arduino Brain Library, Processing Brain Visualizer and the controlP5 Processing GUI Library (required for the visualizer). These libraries, full source code and schematic are provided.