  This paper gives an overview of current trends in manual indexing on the Web.
Along with a general rise of user generated content there are more and more
tagging systems that allow users to annotate digital resources with tags
(keywords) and share their annotations with other users. Tagging is frequently
seen in contrast to traditional knowledge organization systems or as something
completely new. This paper shows that tagging should better be seen as a
popular form of manual indexing on the Web. Difference between controlled and
free indexing blurs with sufficient feedback mechanisms. A revised typology of
tagging systems is presented that includes different user roles and knowledge
organization systems with hierarchical relationships and vocabulary control. A
detailed bibliography of current research in collaborative tagging is included.
