Reference Managers: Zotero and Mendeley

Tutorial	Quick Start Guide	Tutorial Videos
Cost	Free and open source	Free
What do I need to  do?	Create account, download a standalone onto your laptop/desktop, and install plugins in browsers and MS Word or LibreOffice.	Create account, download a standalone onto your laptop/desktop, and install plugins in browsers and MS Word or LibreOffice.
Operating Systems	Windows, Mac, Linux, and IOS and Android	Windows, Mac, Linux, and IOS and Android
Can I collaborate?	Yes, you can share references in your Zotero library after syncing through your account at zotero.org	Yes, through your Mendeley web account.
More help	Zotero Support	Mendeley Help Guides

If these do not meet your needs, please consult these comparison charts from the University of Toronto or Wikipedia. Be aware that some of the tools mentioned on these sites have limited free access. 

Other Popular Citation Tools  : EasyBib        BibMe        KnightCite       EndNote Basic