Define different screen resolution when external monitor is plugged in I have a Macbook Pro that I use both at home and at work. At home when I'm using it as a laptop, the standard resolution of 1440 x 900 is fine as it's closer to my eyes. However, at work I plug in an external LCD so that I can dual screen and the resolution of the Macbook screen is a bit too large for my eyes. 

Is there any way for me dictate a smaller resolution when I have an external monitor plugged in? Or is there an app that does this?
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