Speaking of Drag and Drop, the modifier keys can modify dragging and dropping actions too.  These are most commonly used in the Finder though, to a limited degree, they have similar behavior in other drag and drop situations.

If you grab an icon in the Finder and drag it to another folder, you will actually move that file or application to that new place.  If you actually want to create a copy of that file in the new place you can hold in the Option key while dragging.  A little green ball with a white plus will appear in place of the stem of the mouse pointer indicating that when you let go a copy will be made.  It doesn’t matter if you hold in the Option key before clicking and holding the icon or after you are already dragging, it will work in both cases.

This trick also works when dragging a bit of selected text in a text document open in something like Pages or Mail.  Select bit of text you want to move or copy and then click and hold it to grab it. You May have to click and hold but not move the mouse for a moment so it knows you are trying to grab and not define a new selection area.  Without the Option key the text will be moved to the new destination in that document.  With the Option key a duplicate will be placed in the destination as if you copied and pasted it there.

I should note that if you drag a file or application to a folder on a different hard drive a copy is made by default rather than a move.  This is so you don’t loose something off your main hard drive inadvertently.  The same happens if you drag text from one document to another so as not to damage your source document.

If you hold in both the Option and the Command key while dragging a file or application you will create an Alias of that item.  An alias is a very small file that is actually a pointer back to the original file.  You can launch the original by double clicking the alias or by dropping it on an application to open the original.  This allows you to have the item in two places at once without having two full sized copies of the item.  You can change the name, icon, color and location of the alias, even delete it, without affecting the original in any way yet any changes you make to the actual contents (the text in the document for example) will be the same for both.

You can easily identify an alias by the little curved arrow that appears as a badge in the lower left corner of the icon.  As long as it’s an alias, you can’t get rid of that, it will always be there.  Option Command drag doesn’t appear to do much to text or images in other applications but that doesn’t mean it won’t.  You never know when a developer might decide to use that combination for something clever.