I saw this quote at 37 Signals:
"If I have two candidates in front of me, one that included a cover letter about how he hand-rolled his own blog, comments, and feed aggregator for fun to learn a new framework, and another that just sends a resume with a one-liner in the body of the email, I’m going to be much more inclined to say “hire” for the guy with the cover letter, even if the second guy’s resume is a bit better. Similarly, I’ll be more likely to say “hire” to the Eagle Scout, triathlete developer than a candidate who bludgeons me with all of their “accomplishments”..."
It's from this blog post that talks about hiring practices, and how it says something about the company. It's interesting, and while most companies aren't very sophisticated in their hiring practices, that is changing.
I'm including it here since someone asked me about cover letters recently. They asked if they should still write them.
I say yes. A cover letter is your chance to make a case for why you should be picked. You can highlight an accomplishment, show some of your personality, let them know more about your online brand. They might not care, but why take the chance.
The other thing a cover letter says to me is that you took some time to look at my company more and you want to work for us. You're not likely to blast out 10,000 resumes with custom cover letters. If you do, then you're a hard worker and that says something.
Don't use a template, however. Make the effort to write a letter tailored for the company.
I don't know if it's helped me get interviews, but I've rarely sent out a resume in the last decade and not gotten an interview.