Add Files to iTunes without Copying to Music Folder: "It's possible to add files (audio or video) to iTunes without copying them to the iTunes Music Folder, even when the preferences are set to copy files to this location. Just hold down the Command and Option keys and drag the file(s) to the main iTunes window (not the Dock icon). You'll notice that the plus-sign icon does not appear. iTunes adds the file(s) to your library, but the library's pointers are the files' location, wherever they are.
[kirkmc adds: Dang, I never knew this, and I thought I knew pretty much all of iTunes' tricks!]
Mac OS X Hints - http://www.macosxhints.com