A couple of months ago I was introduced to the Jackson Library IE toolbar, and since then have purchased a toolkit called Toolbar Studio to help me develop a similar one for my distance students (not an endorsement for the product, it's just what I did). It's fairly easy, but I haven't really made the time to work on it much. In the comments section of that post Gianluca Drago pointed towards a toolbar he had made that works for Firefox, and he coded it by hand! Lots of good links on his page if you're up to trying this. And today I learned of another library toolbar that's in the wild thanks to the Digital Reference blog.
Does anyone know of more examples of these?