This is a reference/citation manager with web-based, desktop, and mobile versions. You can sync your citation library between these different versions and across different devices. And it is free. More at https://www.mendeley.com
This is an open-source, Firefox plug-in which does many of the same functions as EndNote and is free. More at http://www.zotero.com
One significant advantage is the ability to harvest citation information directly from webpages, such as Google Scholar or other content pages. The output does export in many formats, including EndNote.
One note of caution, the Zotero plug-in does not play nicely with EndNote downloads, so you need to be aware of this program interaction if you want to work with both programs.