EndNote Web is a web-based citation management tool available to all undergraduate students for free.
EndNote Desktop is a client-based citation management tool available for free to UI faculty, staff and graduate students. It helps you organize references and create properly formatted bibliographies. References can be imported from online databases or can be typed in by hand. EndNote works with Microsoft Word to produce bibliographies and embedded references in hundreds of available styles.
[Free/Easiest to learn]: Mendeley makes it easy to keep your collection of PDFs organized. It takes PDFs, extracts author/title/source information, renames and organizes them, and syncs them to free online storage space. It also includes a PDF reader that allows highlighting and annotations.
[Free/Easy to learn]: Zotero is free plug-in for Mozilla Firefox. It allows you to import and organize citations as well as attach PDFs from your computer. Citations and PDFs can be synced to a free online Zotero account. A Word plug-in allows citations to be inserted into your paper and formatted.