It's tempting to use a stock cover letter that you can send to multiple employers by simply making slight modifications.
However, in the end this wastes time, not saves time. Employers are looking for originality, not banality, in cover letters for resumes. Be wary of using gimmicks or being jokey in your cover letter, but take the time to tailor each cover letter carefully.
Employers will not be anxious to consider your application if you're not willing to put a little effort into your cover letter.