As Bob Ambrogi points out, Casetext is a crowdsourced caselaw annotation website that is mainly effective when people use it. By people, I mean lawyers with relevant work experience. Its like a Wikipedia for caselaw. Since anybody can made citations on Wikipedia, you should probably take those with a grain of salt. Users can annotate cases by tagging them or adding notes.
LearnLeo is a website for case briefing tips for law students. They have a new feature that allows law students to mark up cases to turn them into case briefs. Unlike Casetext, these annotations are kept private. Although this may be useful in a pinch, remember that doing case briefs by hand, or summarizing them in a way most useful to you is the best way to learn them.