Here's a very cool tip we first saw at Lifehacker. Jeff Kelley is a MacBook Pro user who, like many others, connects it to several displays, depending on context and the task at hand.
If you've done this, you may be familiar with the same irritation that has bothered Jeff. Namely, application windows re-sizing themselves across displays. The difference between Jeff and the rest of us is that he did something about it.
Here is a great AppleScript that notices his screen resolution and sizes his application windows according to his likes. Now, Mail, iCal and so on will appear just where they ought to. Very cool!