Zotero is a free, open-source, web and browser-based citation tool. It works best with Firefox, though standalone versions are available for use with Safari and Chrome. Zotero lives in your browser. Features include:
- Free up to 300 MB of storage.
- Good at retrieving PDF metadata.
- Easy import of citations via browser plug-in.
- Automatically imports PDFs when possible.
- Works well with non-traditional electronic sources.
- Good support from user community at zotero.org.