Another somewhat similar service is Diigo, which also has plug-ins for the major browsers that allow you to highlight content in a page (in Diigo's case it becomes yellow) and tag it with keywords. You can also add what amounts to a sticky note with comments, and then save it to Diigo's servers, where it is shared with other users. You can then go back to your account and see not just your own comments or highlighted sections from a site, but also the highlights and comments from any other Diigo user who also saved the page.
