Zotero is free, open-source reference management software. It allows you to collect bibliographic data for sources and then seamlessly create footnotes and bibliographies when you write up your research. You can use Zotero both as a desktop software and also online, and syncing them both lets you access your reference library from anywhere.
Web browser plugins make it easy to grab most bibliographic data and add it to Zotero from the web with one click, and you can organize your sources into folders and subfolders. In addition to bibliographic information, you can also add tags and notes to your sources, as well as identify related sources. Your entire zotero library is also searchable.
Zotero can work with both primary and secondary sources, although it works more intuitively with secondary sources. See links in the "What Others Have Said" section below to learn how others use Zotero to manage their sources.
Zotero also allows you to build and share bibliographies with multiple people, whether for collaborative research, coursework, or public projects. An excellent example is The Syllabus Project's group Zotero library to help diversify environmental history syllabi.
You can sign up for and download Zotero, as well as see all of its documentation and user forums, on its website, zotero.org.
Zotero is free, open-source software, and is a project of the nonprofit Corporation for Digital Scholarship. It works with both PCs and Macs.
Zotero is a very popular citation management software, with excellent documentation and an active user community. Its ability to grab bibliogrpahic information from the web is impressive, although it does sometimes need a little cleaning up. Inserting footnotes and bibliographies in Word is usually painless, but other word processing software (like Scrivener ) can require a workaround.