Wikipedia
A.nnotate is a web service for storing and annotating documents. Documents are either uploaded by the user or fetched from a web address supplied by the user. Uploads are accepted as PDF, Microsoft Word, office formats supported by OpenOffice and common image formats. When a URL of a web page is entered, the service makes a local copy of the HTML and stylesheet. The service offers a browser bookmarklet to facilitate making snapshots of web pages.
Uploaded documents are rendered as images on the server and the images are sent to the user's browser for display and annotation. Annotation with regions and arrows is supported for all documents. For text documents, the server also sends the positions of words on the page allowing the client to offer text search and highlighting. Annotations can be displayed in the right-hand margin, as floating boxes above the text, or as footnotes. For web snapshots they can also be displayed within the main text flow.
By default, all documents and annotations are private. A user can issue invitations by email to allow other users to view and annotate a particular document or to access all documents in a folder. A "reply" option on annotations allows other users to comment on existing annotations offering a form of Threaded discussion. Access controls allow the document owner to specify what annotators may do, including viewing each other's annotations and defining new tags.