GoPro VR Player is a free 360 degree video player for Windows and OS X.
The interface is minimal. A "File" menu has only Open, Preferences and Exit options, the player bar has Play/ Stop, Volume and Full Screen buttons, and that's about it (you don't even get a right-click menu).
This does make for an easy life, though, because the program behaves exactly as you'd expect. Drag and drop an MP4, it starts playing immediately, and you can spin the mouse wheel (or pinch the screen) to zoom in and out, or click and drag to look around.
If you'd like more control, there are some important options in the Preferences dialog. In particular, the Output Projection settings redefine how the video is shown: "Fisheye" and "Little Planet" give you a wraparound sphere-type look, "Rectilinear" is more how you'd see the scene if you were really there.
GoPro VR Player can also recognise and work with your Oculus Rift, as long as your desktop has the appropriate software installed. If there are problems, click Help > Software Log for diagnostic details which might offer some clues.