EndNote Basic is the online-only version of EndNote by Thomson Reuters/Clarivate Analytics. Get up to 2GB of Web storage for file attachments and increased storage for up to 50,000 references. EndNote offers sophisticated, flexible tools for organizing your citations.
Mendeley offers a free version of the reference manager and a social collaboration tool. Mendeley has an integrated PDF viewer for annotation, highlighting and adding notes directly on your PDFs. Create citation records just from importing a PDF file. The free account includes 2GB of online storage.
Zotero is a free open source citation manager that will keep track of your sources and format your bibliographies. Available as a Firefox plug in, or a stand alone application to use with the browser of your choice. 300MB free online storage.