CiteULike (http://www.citeulike.org http://www.citeulike.org) for papers. I like the ability to upload PDFs so that I don't have to keep jumping around paywalls

CiteULike (http://www.citeulike.org http://www.citeulike.org) for papers. I like the ability to upload PDFs so that I don't have to keep jumping around paywalls every time I want to refer to an article.