Tagging - An effective Knowledge Management option?

I have been struggling with figuring out a way to organize all of the materials that I collect and read (from the web, journals etc) in a way that will be valuable and I will be able to use.
It seems that a simple hierarchical taxonomy is not sufficient.
Most documents and articles require more than one label.
I never seem to be able to find something that I know I have, and have recently stopped saving things that I find valuable because I assume that "I'll never find it when I need it".

I am therefore setting up this blog as an exploration in personal Knowledge Management. It could later become a collaboration tool and perhaps even a base for a community of knowledge sharing.