The arXiv is my favorite of the preprint repositories, but focuses mainly on scientific areas. For the social sciences, law, humanities, etc., SSRN (http://www.ssrn.com/) is a decent alternative. Some people are wary of it because it's for-profit (like academia.edu), and it also has a bunch of pointless cruft (such as a completely useless categorization system and proliferation of "top-10" lists), but it does at least allow you to download the PDFs. It's really improved access to legal scholarship especially; like with physics/arXiv, law/SSRN is now a pairing that's widely known in the field and it's basically expected you'll upload your preprint there if you want to be read.