How do you address a letter to a business when it's not to a specific person?

Do you use:
To whom this may concern:
?

When I attended secretarial college - many moons ago - we were told that if it was to, for example, The Managing Director it would be "Dear Sir or Madam", but if it was to a department, for example, Human Resources, you would put "To Whom it May Concern". In either case you need to end the letter "Yours faithfully", as you would only use "Yours sincerely" if you had used a name at the beginning, e.g. Dear Mr Davies, etc., etc.

To whom it may concern:

Gentlemen and Ladies:

as a broker I use " This letter in reference"

I always use "Dear Sir" and hope I don't get a feminist.
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sure you can use, to whom this may concern, or" Dear sir or madam" don't use dear sir alone and don't say dear madam alone unless you know the gender of the reciever. if you don't know wether its a female or male then you say both: Dear sir or Madam.

"Dear Sir or Madam" is best and has had the best responses in my experience. "To Whom It May Concern" is OK too.

for example you are writing to XYZ Company and you want it to be a genera letter or to whom ever is in charge:

XYZ Company
123 East St. Los Angeles
California

To Whom it May Concern

---OR----

Dear Sir/Madam