Annotate This : Allow your readers to add inline comments to any page in your web site or blog and watch your content spread further than you ever thought possible! JumpKnowledge is a free, web-based, zero-install, easy to use, powerful annotation engine that you can now deploy for your own web site or blog. This widget can also replace… (more) multiple social bookmarking buttons because after annotating, your reader has the option to submit it to the leading social bookmarking sites and news sites. Public annotations are also automatically indexed by the leading search engines for even more exposure.

Source : Widgetbox