A web annotation is an online annotation associated with a web resource, typically a web page. With a Web annotation system, a user can add, modify or remove information from a Web resource without modifying the resource itself. The annotations can be thought of as a layer on top of the existing resource, and this annotation layer is usually visible to other users who share the same annotation system. In such cases, the web annotation tool is a type of social software tool. For Web-based text annotation systems, see Text annotation.
Web annotation can be used for the following purposes:
- to rate a Web resource, such as by its usefulness, user-friendliness, suitability for viewing by minors.
- to improve or adapt its contents by adding/removing material, something like a wiki.
- as a collaborative tool, e.g. to discuss the contents of a certain resource.
- as a medium of artistic or social criticism, by allowing Web users to reinterpret, enrich or protest against institution or ideas that appear on the Web.
- to quantify transient relationships between information fragments.
Comparison of web annotation systems 
Many of these systems require software to be installed to enable some or all of the features below. This fact is only noted in footnotes if the software that is required is additional software provided by a third party.
|Annotation system||Private notes||Private group notes||Public notes||Notification||Highlighting||Formatted text||Notes|
|Firefox (built-in)||Yes||No||No||No||No||No||"Description" and "tags" fields of bookmarks provide this|
|A.nnotate||Yes||Yes||No||Yes ||Yes||No||Can annotate PDF, ODF, .doc, .docx, images, as well as web pages (but only a limited number in the free version)|
|Annotary||Yes||Yes||Yes||Yes||Yes||No||Includes social features, following, and a social feed of notes. Private group annotations available in premium accounts.|
|Chatterati||No||No||Yes||No||No||Yes||Currently available as a Google Chrome extension. Allows its users to have a discussion, and vote on each others comments.|
|Crocodoc||Yes||Yes||No||No||Yes||No||Requires Adobe Flash, so doesn't work on iPads and iPhones. Can annotate PDF, .doc, images, as well as web pages. Unusually, Crocodoc offers PDF annotation free of charge.|
|Delicious||Yes||No||Yes||No||No||No||1000 character limit per page per user|
|Diigo||Yes||Yes||Yes||Yes||Yes||No||Public annotations are only allowed for established users. Group tag dictionary feature to encourage tagging consistently within a group.|
|Keeppy||Yes||Yes||No||Yes||Yes||No||Allows annotate web site and stores the quote on the cloud server.|
|Org-mode (with extensions)||Yes||No||No||Yes||No||Yes||Emacs-based; requires technical knowledge to set up; not as user-friendly as some other solutions; non-Latin characters allowed in notes but not in tags|
|rbutr||No||No||Yes||Yes||No||Yes||More of a semantic linkage plugin than an annotation service, but it allows public comments and tags to be associated with rebuttal links. Rebutted pages display alerts which link to the rebutting pages with the comment that connects them.|
|Yes||Yes||Yes||Yes||No||No||Not primarily a web annotation site - public reddits (sections of the site) are mainly intended for new and/or interesting links, but private reddits can be used for anything. Voting up and down for links; links ranked by #votes and age; thumbnails for images and videos in list of links|
|Reframe It||Yes||Yes||Yes||Yes||Yes||No||Compatible with Firefox, IE, Safari & Chrome. Websites can integrate a version of the tool directly into their sites. Allows replies for threaded discussions. Social network integration for sharing & signing in.|
|Scrible||Yes||Yes||No||No||Yes||Yes||Caches a copy of the entire page to personal library along with annotations.|
|SharedCopy||Yes||Development has stopped.|
|ShiftSpace||Yes||No||Yes||Yes||Yes||Yes||Development has stopped.|
|Snowbound Software||Yes||Yes||Yes||No||Yes||Yes||Annotations are XML-based. Available in HTML5 Viewer for Java and .NET as well as in our Viewing and Conversion SDK for Java, .NET, and Windows. Annotate any format supported including PDF, TIFF, Word, AFP, Excel, JPEG, TXT, HTML, and many more.|
|Stickis||Yes||Yes||Yes||Yes||N/A||Yes||Blogs subscribed via Stickis will appear as annotations when they link to the current page. Any web content, including YouTube videos, can be inserted into a note.|
|WebNotes||Yes||Yes||No||Yes||Yes||No||Can also annotate PDF documents (but not in the free version). Highlighted PDFs and web pages can be shared with others, who do not need to install anything to view them.|
Technical details 
|Annotation system||Cloud-based||Technology||Open source||Bulk export|
|Org-mode (with extensions)||No||Text editor||Yes||Yes|
|Reframe It||Yes||Toolbar, Bookmarklet, Website Integration||No||No|
|Snowbound Software||No||Viewer and Toolkit||No||Yes|
Former web annotation systems 
- Annotea - a W3C project that tried to establish a standard for web annotation.
- ThirdVoice - a system launched in 1999 that shut down due to lack of success in April 2001.
- Fleck* - launched in 2005 with much publicity as a stick-it notes application for the web. A patent, funding and marketing didn't stop it from failing. Discontinued in 2010.
- Google Sidewiki was part of Google Toolbar, and allowed users to write comments alongside any web page. It was discontinued in December 2011.
- Dispute Finder was built by Rob Ennals while working for Intel. It attempted to automatically identify disputed claims on websites, highlight them, and link to comments and pages which corrected the dispute.
See also 
- Comparison of notetaking software
- Social bookmarking
- Some reference management software packages also support web annotations
- Online deliberation
- Threaded discussion
- Virtual graffiti
- Comparison of software saving Web pages for offline use
- See A.nnotate notifications
- Yes via the "publish to HTML" feature, but no notifications or discovery of public annotations written by others
- With fireforg, which requires Firefox
- Instead of highlighting a web page, you drag selected content from the webpage into the note.
- via API
- However, because .org files are plain text files, they can be backed up to the cloud at any time, using standard backup techniques/software. Emacs (in which org-mode runs) also supports saving to remote servers.
- Yee, Ka-Ping (2002). "CritLink: Advanced Hyperlinks Enable Public Annotation on the Web". CiteSeerX: 10.1.1.5.5050.
- Third Voice Trails Off, Wired News, April 4, 2001
- Farewell Fleck.com, "The Next Web", May 10, 2010
Further reading 
- Sanderson, Robert; Van de Sompel, Herbert (2010). "Making Web Annotations Persistent over Time". Proceedings of JCDL '10, Joint Conference on Digital Libraries. Association for Computing Machinery. pp. 1–10. doi:10.1145/1816123.1816125.
- Signer, Beat: "An Architecture for Open Cross-Media Annotation Services", Proceedings of WISE 2009, 10th International Conference on Web Information Systems Engineering, Poznan, Poland, October 2009
- On Web Annotations: Promises and Pitfalls of Current Web Infrastructure. Venu Vasudevan and Mark Palmer, Proceedings of the 32nd Hawaii International Conference on System Sciences - 1999
- A Survey of Web Annotation Systems. Rachel M. Heck, Sarah M. Luebke, Chad H. Obermark. 1999.
- New ways of using Web annotations. Laurent Denoue, Laurence Vignollet.
- Ricardo Kawase, George Papadakis, Eelco Herder, Wolfgang Nejdl: The impact of bookmarks and annotations on refinding information. ACM Hypertext Conference 2010: 29-34
- Ricardo Kawase, Eelco Herder, George Papadakis, Wolfgang Nejdl: In-Context Annotations for Refinding and Sharing. WEBIST (Selected Papers) 2010: 85-100
- "Five Ways to Mark Up the Web", Techcrunch April 10, 2007
- "Top Web Annotation Tools: Annotate+Bookmark+Collaborate", MakeUseOf May 13, 2007
- "Protonotes: Annotating Website Prototypes", GigaOM June 2, 2008