The purpose of this post is to tell those people who do not know them.

1. citeulike：it is a free service to help you to store, organise and share the scholarly papers you are reading. When you see a paper on the web that interests you, you can click one button and have it added to your personal library. CiteULike automatically extracts the citation details, so there’s no need to type them in yourself. It all works from within your web browser so there’s no need to install any software. Because your library is stored on the server, you can access it from any computer with an Internet connection.
2. Connotea：Quickly save and organize links to your references. Saving references in Conntoea is quick and easy. You do it by saving a link to a web page for the reference, whether that be the PubMed entry, the publisher’s PDF, or even an Amazon product page for a book. Connotea will, wherever possible, recognise the reference and automatically add in the bibliographic information for you. In Connotea you assign keywords (or ‘tags’) to your references. These can be anything you like, and you can use as many as you like, so there’s no more need to navigate complicated hierarchies of folders and categories. Connotea shows you all the tags you’ve ever used, so it’s easy to get back to a reference once you’ve saved it.
3. [This is not a really online tool, it is only a firefox extentsion!]Zotero：Zotero is an easy-to-use yet powerful research tool that helps you gather, organize, and analyze sources (citations, full texts, web pages, images, and other objects), and lets you share the results of your research in a variety of ways. An extension to the popular open-source web browser Firefox, Zotero includes the best parts of older reference manager software (like EndNote)—the ability to store author, title, and publication fields and to export that information as formatted references—and the best parts of modern software and web applications (like iTunes and del.icio.us), such as the ability to interact, tag, and search in advanced ways. Zotero integrates tightly with online resources; it can sense when users are viewing a book, article, or other object on the web, and—on many major research and library sites—find and automatically save the full reference information for the item in the correct fields. Since it lives in the web browser, it can effortlessly transmit information to, and receive information from, other web services and applications; since it runs on one’s personal computer, it can also communicate with software running there (such as Microsoft Word). And it can be used offline as well (e.g., on a plane, in an archive without WiFi).