lets you search for scientific papers and journal articles, 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 like oaDOI, Unpaywall, and Sci-Hub
See it in action: