I'm currently working on an extension for CodeIgniter called Modele. Modele is just a collection of four files and each file resides in a different directory within the CodeIgniter installation. I wanted to use SVN to control Modele's source code but SVN cannot single out files from within a directory structure. Using SVN for these files meant I'd have to include the entire codebase of CodeIgniter with Modele. That's not the end of the world, but anyone who wanted to check out Modele via SVN would get an entire CodeIgniter install instead of the four files that comprise Modele.
To maintain an SVN repository of only the four files that comprise Modele, I use symlinks. I created a folder called 'modele' in my home directory and copied the four files of the Modele project to it. In the CodeIgniter directory where I'm developing Modele, I removed those four files and created symlinks to the copies that exist in the ~/modele folder. Now, I can import the ~/modele directory to my SVN repository and only source control the files that make up the Modele project.