What's that animal that I hear cooing at night?

In wooded areas, I've gotten used to hearing some insect, bird or other animal making a soothing, repetitive cooing as evening starts. It sounds sort of like "oooh--OH.....ooh, ooh, ooh," with the last few "ooh"s growing progressively quieter. I'm sure this sounds familiar to anyone who's ever slept among nature, but I have no idea what creature makes that noise. Anyone know?