"Dear Madam or Sir" in Cover Letter

When you don't know the name of the person to whom you're sending your cover letter (whether hardcopy through the US Post, via email, or posting it on a job board), never use either of these salutations:

To whom it may concern
or
Dear madam or sir

Instead, use a salutation such as:

Dear Manager
Dear Hiring Manager
Dear Director
or even just
Hello

The idea is to address your unknown recipient professionally without being too formal and old fashioned.