There has recently been a bit of discussion (Tom Coates in particular) about providing browsers with the same kind of tagging offered by the del.icio.us social bookmarking service. I don’t know if it exists in any Windows browser, I know it doesn’t exist in any Mac browser, but it has already been done in two browsers for the GNOME desktop, Galeon and Epiphany.
This dialog appears when the user chooses to add a bookmark using either the menu command or the related keyboard shortcut. The user can alter the label, the address, and choose from existing topics (tags in the del.icio.us parlance) or create new topics.
Epiphany’s bookmark manager allows topic browsing, full text searching, and all the basic editing capabilities you would expect.
This implementation is far from perfect, but it is worth considering as a starting point.