Numerous (incompatible) tools exist for annotating webpages. At the lowest level, a browser bookmark is like a super lightweight annotation: you can put them in a folder, they have the URL and the title, and (in Chrome, at least) that's it. Firefox additionally allows you to tag your bookmarks.
Moving up the ladder of complexity, social bookmarking services like del.icio.us (sadly deceased) might allow you to add descriptions and share bookmarks publicly or privately. Reddit and its ilk fall somewhere around here, offering a place to discuss the pages.
Then, we move into proper annotation tools. Things like Hypothesis, WorldBrain's Memex, or Polar allow highlights and commentary, a fairly minimal feature set for interacting with webpages. Though I say minimal, it's not yet clear to me what richer features might be desirable. Integration with blogging? Some value-add from analysis of the annotations? Collaborative annotation?