We freely admit that there have been many people who've tried to do this, but, to answer your question: Annotator is more granular than Sidewiki (you annotate s

We freely admit that there have been many people who've tried to do this, but, to answer your question: Annotator is more granular than Sidewiki (you annotate specific passages within the page), and unlike Reframeit and Highlighter works across the web rather with no requirement for the developers of those sites to have enabled it.