Binaural Beat Generator
Plays two slightly different tones, one in each ear — the small difference is what creates the perceived "beat". Headphones are required; it won't work over speakers, since the effect depends on each ear hearing a genuinely different frequency.
About this tool
The left ear hears the carrier frequency exactly; the right ear hears the carrier plus the beat frequency. The difference between them is what the brain perceives as a rhythmic "beat" at the beat frequency itself, even though neither ear is actually hearing anything pulsing. Different beat-frequency ranges are popularly associated with different mental states (e.g. low single-digit Hz with relaxation, mid-range with focus), though the research on how reliable or significant these effects actually are is mixed — treat this as an experiment to try for yourself, not a guaranteed effect.