EndNote Online (formerly EndNote Web) is a free and simple, web-based version of EndNote Desktop and may suit some users. You can create an EndNote Desktop library and then sync it with your Online library so that the 2 will always remain the same.
EndNote Online enables you to:
directly import references into an EndNote library
store your pdfs with your references
organise and group your references in a flexible way to suit you
'cite while you write' in the standard Plymouth University referencing styles