SemanticScuttle is a social bookmarking tool experimenting new features as structured tags or collaborative descriptions of tags. More details on the wiki.
fuzzzy is the social bookmarking and networking site for web science academics, web professionals and web enthusiasts. If you do research and/or development of web centric systems then this is the site for you.
SOBOLEO is a web-based system to help users share knowledge and experience more easily. Users collaboratively create a topic list and use this to tag people and web resources. This list is used as background knowledge during searches.