Bunghole is a link indexing system with no sub-categories. Links can be viewed sorted in many ways, has a voting system (listed sites can be rated from their sites as well) that blocks double-voting. Furthermore, it has a newsletter mailing list and comes with a sample database of 300 links (from the Bunghole database). The code is a bit buggy but is an excellent example to show you how to make your own simple link index sites. Bunghole is written in PHP, and uses a MySQL database on backend.