Avoid sending a cover letter addressed to "To Whom It May Concern." If the job announcement instructs that resumes be sent to William Smith, address the letter to "Mr. William Smith" and, in the salutation, write, "Dear Mr. Smith:". Example:
Mr. William Smith
Jones, Jones & Jones
123 A Street
Anytown, WA 90000
[indent] Re: Legal Internship, Summer 2011
Dear Mr. Smith: [that's a colon, not a comma]
If there is no name given, take the time to look up the name of the General Counsel or call and ask the receptionist for the name of the Hiring Partner at the firm.