Many Mac OS X applications allow rectangular selections. That is, selecting multiple lines of text, without having to select up to the end of each line. This can be useful if you have created a text formatted table, and want to select an entire column, or if you want to modify the same thing on every line in a document.
To do this type of selection, hold down the Option (alt) Key while the cursor is over a block of text in a cocoa application. Notice that the mouse arrow changes to a cross.
With the Option Key still held, click and drag to make a selection without the text up to the end of the line being automatically selected when you drag down over multiple lines.
If you want to do something particularly neat, once you have made a selection hold down the Command Key as well as the Option Key, which will allow you to make another selection somewhere else in the document, whilst keeping the first block of text still selected.
This works just about everywhere you are able to edit text, including Microsoft Word (although the cursor doesn’t change to a cross) and text boxes in Safari. As you would expect, it works in TextEdit, Mail, and all the other usual Cocoa Apps.