Who do you address a cover letter to, if you can't find out who to send it to?

I'm sending out a bunch of resumes/ cover letters. I know the names of several of the people I'd like to see my resume, but for some companies, I don't. Who do I address the cover letter to ';Sir or Madam';? Who do I address the envelope to? ';Human Resources?'; Any ideas on how to make sure my resume gets seen?