I'm going to enter graduate school in August and I read in an article title something like "Some number of things I wish I knew before going into grad school" that having a way to keep track of all the papers you've read is endlessly valuable. Incidently I lost the article and don't know the way to keep track of citations and papers. Are there apps or programs that I can use for this? I know Endnote is one but it's also fairly expensive. Is there a method I can just organize folders by?