EndNote is a paid option that costs about $250 to download. Some researchers find that the more-accurate citations, greater array of supported citation styles, and better interpretation of journal abbreviations justify the price.
Zotero is a free "research assistant" that allows you to "collect, organize, cite, and share" your sources right from a bookmark in your web browser. It will sync data in the cloud and across multiple devices, allowing you to work on your research no matter where you are. It integrates with MS Word and Google Docs to help you create and manage in-text citations and bibliography entries.