Many people within my company have asked how they can add links to documents inside their document libraries so that there are not multiple copies of a single document floating around the system.

A quick and easy way to do this is to add a content type of Link to a Document and then use this content type for links.

How to set this up:

In your document library, go into Settings / Document Library Settings

Click on Advanced Settings and ensure that Allow Management of Content Types? is set to Yes.

From the Settings page, click Add from existing site content types

Select Link to a Document, click Add and click OK

From within the document library, select the drop-down arrow next to New and choose Link to a Document

Enter the document name and URL

• You can get the URL from another document by choosing the target document's drop-down menu and selecting Send To E-Mail a Link
• An email will open automatically 
• Copy and paste the link from the email into the URL field on the New Link to a Document form

Click OK and you will see the new document link in your document library view

[Note: Someone has documented these steps with pictures here! http://www.toddklindt.com/blog/Lists/Posts/Post.aspx?ID=49 ]