Doing your Due Diligence is perhaps the most important part of investing in penny stocks (or any stocks for that matter). Finding good sites to study filings, reports and news releases isn't always easy. We recommend these sites:

Finance.yahoo.com - The best, the biggest.

OTCBB.com - Best for looking up information on OTC stocks.

Quote.com - Another comprehensive research site.

Fool.com - Research & articles.

Investorshub.com - Forums and stock research tools.