Comprehensive directories of public companies are listed all over the Web. Here are a handful of good resources:
Edgar Online: Searchable window into the SEC database of companies and filings—as well as company profiles worldwide.
Corporate Information: This site meta-searches, and aggregates data from public companies all over the Web.
Hoover’s: Great vortal, with lots of free profiles and links to public companies.
Start-ups are very volatile and capital is expected to be scarce through the first half of this decade. If you want to reach people inside newer companies who may be looking to jump ship, the best places to find them are at venture capital sites that list their portfolio companies.