Zotero
Zotero collects, manages, and cites research sources. It's easy to use, connects to several web browsers, and it's free. Zotero allows you to attach PDFs, notes and images to your references, organize them into collections for different projects, and create bibliographies using Word, Google Docs or OpenOffice.
Some cases of unintentional plagiarism are due to poor note-taking from sources. For example, students sometimes copy down an author's exact words and forget to put quotation marks around them. Or they accidently mingle their own words with a source's words. Or they've forgotten to note the article title or author and can't find the source again when it's time to document the source in their research paper.
Here are some suggestions for avoiding these problems: