There are millions of different books to read these days. It is hard to pick up a book that doesn't remind you of something else you've read before. This sparks the despairing question: "What can I write that's unique, then?!"
I think the question should be "How can I make this plot unique?" because frankly, the chances of designing a plot line that isn't similar to another person's even slightly is going to be near impossible. But unique writing versus original writing is slightly different, or so I think at least.
A unique plot twists a plot into something that while similar, is still it's own work. Tales have been told many times of the hero who saves the day and gets the girl; Well, what if the girl didn't want to be saved? What if the hero failed, and the bad-guy gets the girl instead? What if the girl is in fact a boy? Twisting the details into a unique tale that veers off the beaten path seems the more likely road to take, instead of making something never before seen.
In a world of genre-fueled writing, this seems even more important. Detective tales are going to involve many of the same elements, so twisting it so things aren't so obvious as they could be help, or playing the sleight-of-hand in writing; keep their eyes over here while you work the magic over there. Foreshadowing (the bane of my readers and my greatest amusement as a writer) is also a good technique, which if done well causes people to read your book again to try to see it.
So instead of despairing about not being able to find an unused plot, why not take one and completely twist it into something unique instead?