Take the time to research a company before committing yourself on paper as a potential employee.
A cover letter should be used as part of a limited, targeted campaign to reach potential employers. Don't send your cover letter and resume in mass-marketing style in hopes of making a potential match.
A successful cover letter should be specific and personal, and should express two important ideas:
What your skills can do for the company.
What differentiates you from the rest of the market.
If you are not sure what sets you apart, consider your background and take note your strengths. Make that your primary focus in writing your cover letter.