Note: This feature is still experimental as of OBS 0.62, so use at your own risk.
If you're using Windows Vista or 7, you probably (hopefully) know that standard Monitor capture performs absolutely terribly if you have Aero enabled, and it is recommended that you disable Aero if you need to use Monitor capture, or avoid it entirely.
However, as of version 0.62 of OBS, it is now possible to use game capture to capture your monitor as if your monitor was a game.
To do so, first make sure Aero is running (Make sure "Disable Aero" is not checked in Video settings). Then, add a new Game Capture source to your scene, and select "[DWM]: Monitor Capture" from the application list. If your monitor is not running at the same aspect ratio as your stream, I also recommend enabling the "Stretch image to screen" option.
And that's it! Preview your stream, and you should now be able to see your Monitor, in all its Aero shininess, with no more performance impact than you would get by capturing a game.
Issues of note:
* This only works with OBS 0.62 and up.
* This method does NOT work in Windows 8, but that's OK, since Monitor capture works well there anyway.
* As far as I know, you can only use this method to capture your Primary monitor.
* You cannot select a subregion like you can with Monitor capture, but you can still crop the source by pressing the Edit Scene button and then holding Alt and dragging the edges of the edit box (you can reset cropping by pressing Ctrl+Alt+R).
* Just like regular monitor capture, you cannot capture fullscreen applications with this method of Monitor capture. You must use a different Game Capture instance for that.
Thanks to HomeWorld for adding support for this!