I just found the Open Source Web Design (OSWD) web site, which offers a collection of reusable web designs contributed by many authors. They currently have 1620 web designs, and all those I have seen are very nice!
My only concern is about licensing. As stated on the web site, the choice of a license if let to each author. Some designs may therefore not be free “as in freedom” or open source, although all must be free of charge (free “as in beer”). So, be careful when chosing a design!