- Scuttle - Web-based social bookmarking system. Allows multiple users to store, share and tag their favourite links online.
- Feed Me Links - Feed Me Links is a web appplication for managing and sharing links. It provides many interesting features such as tagging, comments, rss feeds, tag-clouds, friend lists, a REST API for exporting, browser sidebar, and Firefox & IE favorites import.
- Akarru is a social bookmarking engine, is used to build social bookmarkings sites, like www.blogmemes.com. Users posts links and promote links to front page using voting system. Akarrú es un motor de promoción social de enlaces y blogs, usado para cons
- Scrumptious is a social bookmarking web application.
- TagCloud Perl Module - HTML::TagCloud - Generate An HTML Tag Cloud
- Tag not ed - a system which allows you to create and manage text documents by attaching tags to them. Later, documents can be retrieved by running queries on those tags (e.g., "show me all docs that deal with 'dogs' and 'cats'").
- GNU/Linux Semantic Storage System - Semantic pseudo file system running in userspace as a service, using a combination of Apache Lucene and PostgreSQL accessed via browser. Should scale better than RDB-only applications. Features tagging, saved and live queries, APIs for app integration. GPL, early beta stage but under active development. I put a longer description here.
- http://en.wikipedia.org/wiki/List_of_Tagging_Networks - lots of others