***"Google Scholar" tries to limit Google searches to scholarly sites. Thus, the sources it lists are somewhat more reliable than from plain Google.
The "Business-SPEA Library Research Guides" lists databases by subject.
*** Hoover's company descriptions and data is the best place to look for an overview of a company. It also has SEC filings such as 10-k's, and stock charts for the day, and links to annual reports.
Datamonitor($) is like Hoover's but has SWOT analysis too.
Mergent ($) has financial data on companies over time and corporate histories.
Marketresearch.com ($) has long reports on several hundred products and industries. Market Research Monitor ($) has shorter reports on more products. World Marketing Forecasts has forecasts on product sales.
D&B's Million Dollar Databases ($) has lists of companies, large and small both, for particular SIC codes and locations.
Investext has stock research reports on companies and industries.
MARKETING. Choices II and Mediamark Research Reports, available only in the library, show what kind of people buy which products. Mediamark is easier to use, but less powerful.
JOINT VENTURES AND MERGERS. SDC Platinium, available only in the library, is good for data on joint ventures and mergers.
Google Finance is well-designed for charting stock prices, though it uses FlashPlayer so you need PRT-SCREEN to save a copy.
Yahoo Finance charts stock prices (and you can download the charts) and has links to annual reports, company SEC filings (e.g. 10_K's), etc. It is better than the SEC Edgar site for SEC filings.
Baseline, usable only in the library, has more data and makes it very convenient to use and to draw graphs. It allows comparison of a company's stock price with its industry, interest rates, and such things.
***The White House Economic Statistics Briefing Room has quick graphs and summaries of macro statistics. The White House Social Statistics Briefing Room is similar, but for statistics on such subjects as crime and health.
Lexis-Nexis ($) has a Search Tables section.
The Economic Report of the President has spreadsheets of the tables in the back. This is one source for time series macro data.
FedStats is the government's grand entry page for government statistics. It allows searches across lots of agencies if you're not sure which one has the statistic you need.
The US Government Printing Office's GPO gateway has a searchable U.S. Code and federal regulations, congressional bills, and the U.S. budget.
Economagic has government statistics listed in convenient form. Also, it is set up to let you download data or to run regressions and make charts in a style different from Excel ones online. If you just want spreadsheet or text files of the datga, the St. Louis Fed is good for macro time series data, and perhaps easier to use.
The U.S. Census Business Statistics site describes industries by enterprise size, and enterprise births and deaths.
The Census Bureau has a NAICS site for how to classify industries by number, and how NAICS maps onto the old SIC-code system it replaces.
Some Economic Census data on number of establishments (not firms) by industry and location is conveniently available on the Web in tables and for download in csv format, and other data is on a CD ROM. See the Guide to the 2002 Economic Census and the 2002 list of data items. This is where data on concentration ratios and on on industry costs are located.
The Bureau of Labor Statistics tables for unemployement and prices. Federal Reserve data includes national wealth.
State and County QuickFacts is where to find data by state and county. The University of Virginia has a nicely organized site for the County and City Data Books (1994 and 2000 data)
***The World Bank has a "Doing Business" site giving quantitative evaluations of regulations in different countries and links to their laws.
The Penn World Table, has international macro data set up very conveniently to create tables by year or country.
The OECD has data on 30 industrialized countries. See its Frequently requested statistics. The CIA World Factbook has maps and facts about different countries.
The World Bank's data query has data from recent years on GDP growth, foreign investment, mortality, etc. by country. It also has massive files of bilateral trade data that are not so user-friendly.
Jon Haveman's International Trade Data (now maintained by Raymond Robertson) links to lots of sources.
The Economist Intelligence Unit ($) has good "Country Commerce" reports on doing business in different countries.
The RDS Business Reference Suite($) is the best source for trade journals, and has a good set of menus for searching by concept ("ad budget") and industry.
"Indiana University Databases," mentioned earlier, is well organized for finding particular magazines or other data souces. Look there for links to ABI-Inform ($) and EBSCO Business Source Supreme, which are similar except for covering different business periodicals; to Lexis-Nexis ($), which covers both legal and nonlegal periodicals, and to Factiva ($), the Dow-Jones database site, which is good for Wall Street Journal articles.
Bloomberg, available only in the library, is especially good on biographies, and it has lots of interviews, including sound files and video.
The U.S. Code (Cornell) has federal laws-- for example, 18-1001, the law against false statements to FBI agents that Martha Stewart broke.
The Cornell Law School collection and Supreme Court opinions is a free reference for law. Findlaw.com has links to court decisions, and is especially good on current legal issues.
***Opensecrets.org, has campaign finance information, including data nicely arranged by industry
The Brookings Institution and the Economic Policy Institute (liberal)
The ***Heritage Foundation (whose reports I find particularly well organized) and The American Enterprise Institute (conservative)
The Cato Institute and the Reason Foundation (libertarian)
CIAO ($) has lots of think tank papers, including ones not available freely on the web.
***Google Book Search searches the full text of many books, in huge scanned-in pdf's. Project Gutenberg has plain-text editions of classic works.
***Copyright Clearance is a centralized place to buy reprint rights.
***Google Tips (for details, go to Google here ; see also my Google tip page):
The Historical United States Census Data Browser is nicely put together. You can find the number of slaves in Connecticut in 1790, for example.
Many Gallup polls are available.
New Yorker cartoons are searchable here, and you can buy permissions.