Zotero is free, cross-platform software that is used to collect your research in one place. Zotero [zuh·teh·row] not only collects citations, but also PDFs of full text articles when possible. It allows grouping of research into searchable collections while adding your own notes and tags. It can then export a bibliography in a variety of styles for your research paper!
Zotero can be also used on Android, ChromeOS, and iOS devices by using an in-browser bookmarklet. This does require a Zotero account.
Zotero - Downloads, documentation, and creating an account (a free account is not required to use Zotero, only to use Zotero cloud storage)
ZotFile - Free Zotero plugin that creates Zotero notes from PDF annotations and provides batch import/renaming operations.
ZoteroBib - Helps you build a bibliography instantly from any computer or device, without creating an account or installing any software.