You can use iGlasses to play a video or show a picture "into" a video app instead of the image from your webcam.
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This can be useful for showing a video or picture to someone on the other end of a video conference or video cast, or maybe to pretend to be someone else for a while.
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If you look on the Effects tab, on the last page of effects, you’ll see “Share Media.” This will let you play a movie in place of your camera. Now you can share a video clip or picture from within any program. For example, you could play a movie directly into Skype, or a Google+ Hangout.
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There is a play/pause button and a scrub bar that works like QuickTime Player’s controls. This lets you have complete control over playback.  
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There's a also a picker for Recently shown files. Click the x in the corner of the thumbnail to remove the image.  
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As a shortcut, you can also just drag a movie file's icon into the effects section of iGlasses to play it instantly. You can also save an iGlasses Preset for a movie.
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There’s no way to hook the audio from the movie directly into your video chat, but if you turn on your speakers and microphone, your fellow conference partners should be able to hear the sound from the movie feeding back into the microphone.