MacBook – External monitor CPU usage

You don't need any apps, Snow Leopard and Leopard (and possibly earlier versions) do this out of the box. I just tested it and it worked without any effort at all.

• Go to System Preferences > Displays
• Select the checkbox 'Show displays in menu bar'
• Connect your external display
• Arrange them however you like
• On your menu bar, you'll see an icon shaped like a screen with two legs, click it
• Set your resolutions and that's it

When you disconnect your external screen, your MacBook screen will return to the original display setting, if you plug the external screen back in, the resolutions you setup last time will be automatically selected.

Note: I tried this with a Mini-DisplayPort cable because that's all I have. I don't know if behaviour is different if you use a different connection. I'm on a 13" Unibody 2009 model and running OS X 10.6.6

You have a solid troubleshooting technique, so I'd agree the adapter, or the VGA cable are likely culprits. Sometimes it's as easy as the cable not sitting well in the connector, and that could mean it connects the necessary pins for detection/sync but not for actual display.

One other thing I could think of is if the monitor has a source selector, i e if it has multiple inputs for VGA/DVI etc, and the right one isn't chosen. That's probably not the case though as you say it responds to resolution change.