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EndNote Basic is the free, web-based version of the program. It allows you to store 50,000 references in the cloud and provides 2 GB of PDF attachment storage. 21 citation styles are available, and you can download a plugin to use your library in Word ("Cite While You Write"). Sign up at myendnoteweb.com.
EndNote Desktop provides complete access to the program's features and will sync across multiple computers and to your EndNote Web account. "Cite While You Write" is included. Storage space is unlimited and there are over 6,000 available citation styles. The desktop version allows you to annotate PDFs directly within the program. You can also share your library and customize your own citation styles. There is a student discount, which makes EndNote about $120. You can try out a free 30 day trial before deciding to purchase.