This article is split on this - 52% of marketing managers dislike off-the-wall tactics BUT 46% of advertising managers say it's OK "provided the style doesn’t detract from the information."
The point is to make sure you stand out in a way that's appropriate for the job you want.
Whatever you do needs to communicate to the employer how what you have done relates to the open position, and how hiring you will enhance the employer's ability to achieve their goals and mission.
Remember that you are communicating with a person, and not with an organization. All cover letters need to be written with a person in mind. Cover letters can be a little folksier, more colloquial than a resume.
That's what direct mail writers do, and what I did successfully for many years raising money for City Harvest and other non-profit organizations. I worked to convey our work in a personal way, so that it touched the person reading the letter.
So your cover letter is your chance to convey a little personality. Here's a great quote by Arlene Vernon, an HR consultant, trainer and speaker, from this article:
“Every so often I get a glimpse of someone’s personality and sense of humor by how they write their cover letter. I have to admit...I appreciate that they’ve gone out on a limb to set themselves apart from the others. And I am sure to read their resume somewhat slower than the 30 seconds I skim for primary content...[but w]hen it comes down to selection, I still base it on work history, writing skills (cover letter), length of tenure in jobs and job knowledge when selecting who to connect with.”
You also can include something in your resume that might stand out as well.
* I remember putting in a resume that I helped launch ZooDoo, a compost product. It definitely got people's attention and was usually one of the first things someone asked about.
* Recently a client got an interview, and the person asked her almost immediately about her job years ago as a steelworker in Chicago.
A little personality and quirks can make you more interesting - as long as you meet the basic qualifications.