Annual Report Gallery
Downloadable and printable annual reports from Fortune 500 companies (though some only in Adobe Acrobat format)

Companies Online
Search through 60,000 companies with web presences, public or private, by industry, name, geographical location, and, with free registration, by sales, number of employees.

EDGAR
Search SEC filings of public companies

Fortune 500

Fortune Global 500

Hoover Stockscreener
A marvelous tool from one of the great business information publishers allows you to choose any or all stock exchanges to search by any or all industry groupings for stocks that meet up to 20 investment criteria. The resulting list of companies is clickable. For each company, you can get a profile, then click for SEC filings and historical information (and more info yet if you're a subscriber to Hoover's).

Moody's Online
Even without a subscription, there's a fair amount of free reports and other financial data on companies and industries here.

NASDAQ
Search latest stock data by company name or ticker symbol.

Standard Industrial Classification Scheme:
S since much ranking of businesses is done by SIC, here is how to find out what the SIC number is for a company's product or service 

Yahoo Finance
Company index. profiles by industry, stock tickers, etc.