After reformatting and losing my bookmarks of posts on another forum I visit frequently, I have had this idea. (Note: I now bookmark with del.icio.us)
What if on each post there was a button that said "Bookmark This!" Now, instead of actually bookmarking the page, it brings you to another page which asks for an optional "Why?" field. You fill that in, the URL/Title is already filled in and then click "Bookmark!" It stores this bookmark in the database, and you can view it in your User CP. Maybe the ability to view others bookmarks, but if you were to add that you would need to have the ability to mark bookmarks "private." Kind of like a local del.icio.us, sans-tags. Although, tags would be nice
The programming wouldn't be too intensive. A four field table,
user_id | url | title | why
INSERT query for marking as bookmark, SELECT query within a WHILE loop for reading your|others bookmarks.