I'm an undergraduate and about to apply to graduate school. I did summer research that contributed to a paper and was asked to extend my time there to write the publication but could not stay due to academic reasons. Because I contributed to it, is it appropriate to include it on my CV?
I had a similar situation, there were a few papers I assisted with as an undergraduate where I was listed in the acknowledgements but wasn't listed as an author. Generally, people don't list those on their CV, simply because it will look odd... you'll get a lot of people asking, "why are you listing this if your name isn't on it?"
That said, since it's your CV, you can list anything you want with the proper commentary. You may consider listing it as:
<Citation goes here>
Note: I am listed in the acknowledgements section for this paper, as I assisted extensively with data preparation and analysis.
Do realize that the fact that your name isn't on the paper will significantly decrease it's value in your promoting yourself. The more I think about it, it's probably a bad idea to list it... I would recommend just listing the work you did in the appropriate section on your CV and leave the paper out.