Remember, it's all in how you market yourself!
Writing a paralegal cover letter is important. A resume is basically an outline of your career so far. A cover letter gives a more personable account of your career and does something else ... it shows off your writing skills. It's more of a short essay than an outline. And in the paralegal profession, good writing skills are important.
An important point of any cover letter is to write it for the particular position you are applying for. It's even more important to do for your cover letter than for your resume. If possible, find out who will be reading your resume and cover letter and include their name in the salutation. Be sure to put down your contact details in the address section and write out the correct address to the law firm or corporation you are applying to.
In the opening paragraph, you will need to clearly convey the exact job you are applying for. You may want to state something like, 'I am writing to you in response to the Paralegal Position advertised in the Washington Post on Jan 5, 2007'. Make sure you make the position you are applying for very clear (often, there is more than one and person reviewing for candidates is not going to want to figure out which one you're applying for).
In the following paragraphs, make sure to point out how your experience makes you a great fit for their position. Use examples from your career so far. Then in the closing paragraph, state that you have enclosed a copy of your resume and ask for the interview. Tell them you are looking forward to hearing back from them and be sure to include current contact information.
Just like with your resume, your paralegal cover letter should be reviewed by a few trusted collegues before you send it out.