SSRN Economics

NBER

Working papers by major researchers. Many of these papers get published in peer-reviewed journal

IDEAS RePEc

The largest database of economics publications (2,000,000 items). Searching through papers is easier with Google: site:ideas.repec.org <search term>. Index sources mentioned below

Google Scholar

Searching academic literature in general. Features author pages and citation counters. If you look for economic writings only, IDEAS would be more powerful