Some social sites like webs, spruz, and grou.ps allow a free for all link (FFAL) link index where you can add categories to it and then thumbnails and a link of a website. I am currently using webs to be my site's link database, but It is a pretty cool feature and you really can't find them anymore on the web where they are remotely hosted.
I've thought about using a phplink directory script as well but the templates for the free version is rather ugly.