I'm currently working on a bookmarking plugin (screenshot attached, WIP), and also
plan to allow users to add their own bookmarks. I'll use the "Flash Cookies" for now,
which means they will only be available on the same machine.
But if you have someone at hand who knows Flash it should be fairly easy to change
it so that it's sent to be stored in a global database persistently for logged in users
(if you'll have something like that).
The graphics will be completely customizable via ASUL.