Reference Manager Software

I have been working with a few referencing software. I am yet to master one! I find new features in each of them. I am interested only in open source version. The reason is obvious. I don't have enough penny to purchase the commercial ones. Some of the software that I am working include Zotero, Mendeley, JabRef, CiteUlike, Bibme, RefWorks, Referencer(Linux), Connotea, etc.