Much more than a bookmarking application, it is a true document marker, recognizing songs in iTunes, web pages, Text documents.
Hallon can create bookmarks in many applications (like iTunes) that usually can't create bookmarks and they are always accessible from a menu whatever you are doing. You can e.g. bookmark your favorite songs, an email that you have to reply to later that day, some documents that you are working on or the contact information of a friend whom you need to call before the end of the week.
And it's open source. Get it here.