A topsite website ranks a group of related sites by popularity. Websites join and add a link back to the topsite. There are several scripts available to start a top site both free and commercial.
Aardvark Topsites PHP is a free topsites script built on PHP and MySQL. It is licensed under the GNU General Public License.
VisioList is a commercial topsite script that is more advanced than AardvarkTopsitesPHP.