Recognize already saved items
Zotero should tell me, when i'm on a page of an article, whether i've already saved it, just like the yellow star tells me whether a website is already in my bookmarks.
Right-clicking the url-bar button should have an item for "show me this item in my library" for easier navigation.
Right-clicking the url-bar button should have an item for "show me this item in my library" for easier navigation.
This won't happen on detect - i.e. the equivalent of the yellow star - since it requires more than simple URL matching and thus would slow down Firefox. It could happen on import, though.
See the thread above for a workable solution on FF and some obstacles for implementing this more broadly.
However, i understand that this would not cover all items and not all resources (e.g. items added from pubmed would not be recognized on google scholar), but it would be better than nothing and it could be some kind of pre-detection (no need for further checking if a url matches).
Edit:
False positives shouldn't be a problem, just let the user decide. Same window as used for synchronization questions.
But if false detects in both directions are too common - as they would be just going by URL matching - this becomes worthless.
http://www.zotero.org/support/plugins
What's useful to you depends, of course, I think everyone should have Zotfile installed, everyone with a Mac or Linux machine should have quicklook, people who like keyboard shortcuts should have Zutilo, most of the others are more specialized.