The best way to keep track of your references is to keep a record of the articles you have read. Reference management tools help you organize your references and automatically generate reference lists or bibliographies in a variety of styles.
If you are going to keep records of large numbers of references then it will be worthwhile investing time in selecting and learning how to use the tool. There are several free web-based tools. Some are basic tools for undergraduate study, others are more advanced and suitable for researchers and postgraduate students.
Basic tools
BibSonomy is a social bookmarking service and can be used to export bibliographies in a variety of formats and feeds.
RefME is a web-based tool for generating citations, reference lists and bibliographies; there is an option for importing .ris files. It is also available as an App.
More advanced tools
RefWorks Simplifies the process of research, collaboration, data organization, and writing by providing an easy-to-use tool for citation, bibliography, and reference management.
EndNote Basic is entirely web-based and allows you to store, manage and share your references, cite sources and write your papers. It is a simpler, free version of the commercial tool Endnote Desktop. (Please disregard the 'Sign in via your institutional login' option on the 'Sign In or create an account' page.)
Mendeley is a reference manager and academic social network that is fully searchable with capacity for reading and annotating PDFs on any device. It is available as web-based or as a standalone desktop version.
Zotero is a free bibliographic software extension for Firefox. It is also available as an extension for Chrome and Safari browsers, and as a standalone desktop version.