You're preparing to give a presentation. You load up PowerPoint (or Prezi, or Impress), load your presentation and get everything just right. When it's your turn, you step to the podium and plug your laptop into the projector -- and everything goes nuts.
The problem is that upon seeing the projector (or other video device) your computer changes video modes to match. Maybe it mirrors your screen to the projector. If so, it may have to change video resolutions. Or maybe it makes your desktop span both monitors. Either way, you end up messing with your settings for a minute or so and your smooth introduction to the audience is blown.
EM-EDID-HD15 by Hall Research.
Before the presentation I plug this in. Since the laptop thinks it now has a second display, I can get the video mode right -- mirroring, resolution, etc. Then, when I plug the projector into the other end of the emulator, the computer doesn't know anything has change and the presentation goes up smoothly.
As of this writing, the EM-EDID-HD15 is available from Amazon for $122. Google shopping shows some much better prices.