We just rolled out a brand-new feature for Citationsy:
Citationsy Archives lets you search for journal articles and papers, download them, and of course cite them in your Citationsy projects.
After entering a query it searches through all published papers in the world and shows you the matches. You can then click a result to see more details, and immediately cite it from there.
It will also let you download the paper through a couple different open access services, or analyse it with Scholarcy
See it in action: