It might not seem like so much of a problem, but if you have a key category that has a single product, then taking your potential customer to a listing page to choose from a list of one looks a bit rubbish!
This little module solves the problem nicely by adapting behaviour so that all single product categories divert the customer directly to the product page. It looks slick, reduces customer annoyance and increases conversions. Best of all it is completely free!
More details can be found at www.magentocommerce.com/extension/1455/single-product-category-redirect