Since more than 1,000 hundred organizations and individuals have put their economic working on the Internet, it is no longer practical to list each site here. Rather, you should check with the sites listed here that specialize in working papers.
The broadest is RePEc (Research Papers in Economics) which has some 800,000 papers and publications in its database. But, it is a "back-end" system not designed for end-users. However, the database can be accessed at IDEAS (Internet Documents in Economics Access Service) and EconPapers and other sites. The Economics Working Paper Archive (EconWPA) was the single largest independent collection of working papers in economics; its contents are in RePEc, but it hasn't accepted new papers since 2006. The very-well established National Bureau of Economic Research (NBER) Working Papers are available on-line, and their "meta-data" is also in RePEc. Two entities here are not in RePEc; AgEcon Search is clearly specific to that field and Economics Research Network (ERN) , part of the Social Science Research Network (SSRN), has some 300,000 documents in all the fields it covers.