I fell in love with Mendeley first, but moved to Zotero for some reasons that are too hard to explain here. Both are great, but I think I'll go back to Medely after reading this. I've always liked their UI and group features better, as well as their metadata extraction process.
Plus now they're giving 5GB of storage with the free account.
http://blog.mendeley.com/start-up-life/team-mendeley-is-joining-elsevier/?utm_source=feedburner&utm_medium=feed&utm_campaign=Feed%3A+MendeleyBlog+%28Mendeley+Blog%29
