Zotero is a free, easy-to-use tool to help you collect, organize, cite, and share your research sources.
Learn more about Zotero from their own quick start guide.
Or watch this introductory video:
Zotero is available for Mac, Windows, and Linux. Even if you use Firefox, I advise you to use the Standalone application (which can still be used with Firefox). Zotero as a Firefox extension will likely be unsupported in the future.
Zotero's interface has three basic columns.
The first column on the left displays the various collections you've created. Collections are folders and subfolders where you organize your research.
The second column in the middle displays the items that are in the selected collection in the first column. In the picture above, I have My Library selected, which displays all the items from all my various collections.
The third column on the right displays the information of an item that is selected in the second column. This column has various tabs at the top for different information attached to that one item. The Info tab is where the bibliographic information of an item is shown. You can edit all these fields in the Info tab for accuracy.