Useful sites for databases

Yahoo! FinanceReal time and historical stock price information along with other up-to-date metrics
Google FinanceReal time and historical stock price information along with other up-to-date metrics
FinVizThe most comprehensive free screener on the internet
SSRNComprehensive scholarly journal archive with a focus on social sciences (i.e. business, finance and economics)
SEC EDGARThe SEC's official filing database. All SEC filings from every public company can be accessed here
Trading EconomicsDatabase of FOREX and trade-related metrics
DataromaUp-to-date information on positions, commentary and recent trades from major institutions and hedge funds
Whale WisdomSite that aggregates 13F filings from institutions and hedge funds
StockMapperMarket-wide heatmaps
MorningstarThe leading mutual fund screener
H.W. WilsonDatabase of over 800 business journals
FREDThe Federal Reserve of St Louis' database of over 72,000 economic timeseries
The Economist Intelligence UnitThe Economist Group's database of reports and forecasts on countries and various economic issues
Bloomberg TerminalsDatabase of real-time pricing information, metrics and research reports for a wide range of securities
MultplSimple way to view several key market-wide metrics such as the PE ratio of the S&P 500