Audio / Field Recorders
Field recorders are designed to record audio separately from a video file. While this makes them slightly more difficult to work with, as they need to be combined with the video later in post production, they make up for this by recording better quality audio than you can achieve in camera.
They also often have multiple inputs, allowing you to record audio tracks from different sources on the one device, and some come with their own built-in stereo microphones.