While Preview's bookmarks and search features are great for jumping between different pages in a PDF document, sometimes it'd be handy to view more than one page of a single document on the screen at the same time. Unfortunately, Preview (like most Mac OS X applications) lets you have just one instance of a file open at a time. But there's a way around that restriction.
The open command line program accepts a parameter -n that allows it to launch the application again, even if it's already running. So, if you're currently viewing file.pdf in Preview, enter open -n file.pdf in Terminal. (You'll need to supply the full and complete path to the file, of course; the easiest way to do that is to type in open -n, then drag the file from the Finder to the Terminal command line.)
Another instance of Preview will launch, complete with its own Dock icon, with the PDF document open. Unfortunately, some applications (such as iTunes) resist even this technique, so you cannot launch a second iTunes instance--to, say, have multiple iTunes libraries open at the same time.
If you wish, you can put this command into an Automator service, making it that much easier to launch. Open Automator and select Service from the template list. In the bar near the top of the editing pane, configure the new service to receive PDF files in the Finder. Open the Utilities section in the Library on the left, then double-click the Run Shell Script action from it. In the editing pane, select As Arguments from the Pass Input drop-down menu. Replace the contents of the action's text area with:
for f in "$@"
open -n "$f"
Save and name the service--Open Selected PDFs in New Preview, for example. Now, when you want to open a Preview document the second time, you can select it in Finder, and then select your new service from the Finder -> Services submenu (or from the context menu that appears when you right-click on the file).
You can, of course, adapt this service to work with other apps and file-types. Just remember that some applications cannot handle multiple open instances of the same document; generally, use this only for viewing, not for editing.