Khulisa finds Mendeley to be one of the best free referencing tools because of its user-friendliness. Mendeley is a desktop application that has a MS Word & LibreOffice plugin as well as a web browser extension for easy online referencing. Mendeley is available for Windows, Mac & Linux.
Other useful free reference managers are Zotero and Docear. Docear offers a useful mind maps function to help you manage your data. The School of Computer Science & Statistics at Trinity College Dublin has provided this comparison between the three online referencing tools.