Tags for Tabs/Groups/Nodes to make non-hierarchical organization possible

Often it is quite hard to organize information hierarchically, because many tabs/nodes do not clearly fit into just one part of the hierarchy. This is especially problematic when collecting things like whitepapers, talks and other scientific articles that often touch on multiple topics at the same time, and organizing them is a pain. Finding an exact hierarchy by which to sort information is most often impossible, and even finding a reasonably good one is a difficult task.

I would propose being able to tag nodes with arbitary tags, and add a way to search/filter the whole tree by those tags. By this an option of non-hierarchical structure would be enabled, complementing the current hierarchical system. This could increase productivity for power users with huge trees by a lot.