The best cover letter I wrote, I actually thought I might've been going insane while I wrote it. I was applying to quite a few jobs at the time and it started to feel like I was just spitting out the same cover letter over and over. I read a job posting that struck a chord for some reason - I think it was that the posting didn't seem quite the same. It had some more colloquialisms than most and just funny terminology for things, so the cover letter I wrote was very laid back. I recall specifically that they wanted a stage manager that could "put out fires." At the time, I was working on a cruise ship and had just completed a marine fire fighting course, so I put as a P.S. something like "and as for putting out fires, no worries here, I just aced a marine fire fighting course."
I sent it off and thought I would never hear back from them, or all the humor I'd put into my cover letter would be lost. I got an interview and a job offer. I wound up not taking it, but it certainly gave me a new perspective on cover letters. Not that that's always the best approach by any means, but it did make me wonder how many opportunities I'd missed by sending out "safe" letters.