Playing nice with Windows, episode 1: Extensions

Part two - Adding Extensions (in which we tweak Mac programs that are stupid - Office, I'm looking at you...)
In this section, we will demonstrate how to add a file extension when saving a file from an application such as MS Word 2004 for Mac.

Create your Document.

Choose 'File -> Save'.

In the 'Save' dialog (as seen below), check the box labeled 'Append file extension' if it isn't already checked.

Notice that Word will then automatically add the appropriate extension, in this case '.doc'. Depending on the format you save in, the extension may change.
This works the same in most Office applications. Keep in mind that not every program recognizes and attempts to compensate for it's lack of common sense.