For the last couple of days, I was searching for self-hosted bookmark management software and found a great solution that is under active development: SemanticScuttle.
I tried to install Pligg but I experienced too many problems, checked out PHPDug, Scuttle, Hotaru CMS but these are not actively developed. There is also Maneame but it is in Spanish.
This blog post is now obsolete, it is only for historical purposes.