I'm currently using del.icio.us but I think this applies to any bookmarking tool. I'd like to see when I have already bookmarked another page on the same site. Why? The main reason is to prevent multiple bookmarks differentiated only by irrelevant parts of the URL (e.g. with/out the index.html). Are there any tools (ideally a Firefox plugin to help with this?
When I was using Google bookmarks I like the Google toolbar feature that shows when you are on a page you have already bookmarked. I found a Greasemonkey script called Familiar Taste that does the same thing in a pop-up window for del.icio.us bookmarks.