A: Yes, you do.

If you're responding to posted job openings, either by email or snail mail, or hand-delivered to the company, you need a cover letter accompanying the resume. The cover letter provides the vehicle to identify the position for which you are applying, and highlight and expand on relevant information from your resume (or even more importantly, identify skills and strengths not mentioned on your resume that would positively impact your performance in that position). In addition, the cover letter gives you the opportunity to explain tricky situations like changing careers or a long and obvious absence from the workforce--and to do so in a manner that shows energy and personality.

If you're targeting hiring officials within companies that have no posted job openings (but might in the future, or might create such an opening if you're sufficiently persuasive), you need a cover letter to help steer the inquiry.

And yes, even when posting your resume to a job board (i.e. Monster), a cover letter is still recommended.

When don't you need a cover letter? There are a couple of instances when a cover letter is not necessary: 1) when hand-carrying your resume to a pre-arranged interview, and 2) when delivering your resume to a recruiter who has reached out to you. OK, let's add one more: 3) when the online application form for a posted job opening either specifically says "no cover letters" or doesn't provide an opportunity to include one with your resume.