There are quite a few efforts out there. Someone has already mentioned rbutr (actually my personal project) - you can view it in action without the plugin by just adding rbutr.com/ to the front of any URL, eg: http://rbutr.com/http://www.realfarmacy.com/johns-hopkins-sc...
However rbutr isn't really what you've described here. What you've described is either
Oh, also, Hypothes.is had previously put together this spreadsheet on Google Docs which lists all of the previous efforts to make web annotation applications. It hasn't been kept up to date though so I keep a local one up to date now, just in case this is ever lost: https://docs.google.com/spreadsheet/ccc?key=0Aujm_HldNh4WdHJ...