EndNote Web is a freely available web-based citation management tool. It allows you to store up to 50,000 references and up to 2 GB of pdfs. it allows you to Collect references by searching online databases or importing your existing collection; Organize and group references in any way that works for you. Then share your groups with colleagues; and Use a plugin to format bibliographies and cite references while you write.
Mendeley is a citation management tool that allows you to Add papers directly from your browser with a few clicks or import any documents from your desktop. Access your library from anywhere. Windows, Mac, Linux and all browsers. It also allows you to Generate references, citations and bibliographies in a whole range of journal styles with just a few clicks.
Zotero is an open source, freely available desktop and web-based citation management tool that allows you to manage citations, create bibliographies, and import references from databases. It can convert stored citations into formatted bibliographies. Formatting styles include APA, Chicago, and MLA (among others). To store your citations, you are required to register with the site. Creating an account is free.