These are probably the most popular and the best wireless spy listening devices. You must have seen it in movies, the spy agent leaves a small device in the car and following them from behind he is able to hear the entire conversation. This small device is known as a bug. A bug can look like anything. It can be a lighter, a wrist watch, clock, etc. The best part about this device is that you can not only listen to the conversation but you can also record it at the same time. Hence, gives you the freedom to store the proof. Few of the advance bugs come with specialized recorders, which records voice only when on phone or while talking. Hence, it does not record all that unwanted silence.