For PCVR users, that's likely going to be DeoVR. SteamVR has a built-in media player, but DeoVR is far better. More flexible, lets you move the video around, etc. Like, one of the big things for me when doing VR porn is that the camera tilt needs to match my head tilt, or else I get motion sick. So if the camera is pointed down 45 degrees, then by default, that means if my head is pointed straight forward, my view is actually looking down 45 degrees, and I get sick. DeoVR lets me move the video down, so that looking downwards makes me look at the center of the video.