Scuttle is an open-source clone of del.icio.us, Yahoo's popular bookmarking site. One of del.icio.us's strong points is its API, which allows for 3rd party plugins to enhance the experience and interpret the data into mashups. Scuttle has replicated del.icio.us's API, and there is even a 3rd party Firefox extension, Scuttleicious, which enables posting to any scuttle site from the toolbar.
Scuttle is disturbingly simple - there is no admin or god mode available. All users are simply users - after installing Scuttle one simply creates an account like any other visitor. This is both satisfying and frustrating, depending on the level of customization you require. Its simple to change the title and easy enough to make a new layout / css file, but you're on your own. Customization beyond this seems difficult, but with time its certainly doable.
Scuttle is perhaps most useful as a private social bookmarking server. By using the watchlist feature, one can share bookmarks only with a select group of people. Although this goes against the idea of social information, its a pragmatic solution for a real need.
The software uses PHP and MySQL, making it compatable with most shared hosting. Its distributed under the GNU GPL, so it can be used in commercial products.
Homepage: http://sourceforge.net/projects/scuttle/
Download: http://sourceforge.net/project/showfiles.php?group_id=134378