The cover letter I read today illustrates a number of basic grammar mistakes. The overall impression is either the job seeker can’t write or has no attention to detail – possibly both. This is the cover letter:
Iam purusing a Project Manager of Construction job. With multiple years of experience and millions of dollars in residental and commercial developments being seccessfully completed you will see in my resume that iam qualified to assist any company with their projects. I look forward to hear from you in the near future.
There are a number of typos. Purusing should be pursuing, residental should be residential, seccessfully should be successfully and hear should be hearing. This are basic mistakes – probably the result of typing quickly and not proofreading. What I don’t understand is the use of “Iam” instead of “I am.” It’s used twice in the letter, leading me to believe the job seeker meant to type it this way.
The problem with a cover letter like this is that it creates substantial doubt about the job seeker’s credibility. This individual listed their salary requirements – $90k and up. Do you think a company is going to be comfortable paying that high of a salary for a manager that could be responsible for decisions on multi-million dollar projects?
Below is the same cover letter in Microsoft Word. As you can see, almost every mistake is caught by the grammar check and underlined in red. 
The one mistake the grammar check didn’t catch is “hear” instead of hearing. Software grammar checkers are not perfect, but they do help in avoiding a lot of mistakes. A cover letter and resume are so important to your search, running them through spell check is essential. One minor typo will often be overlooked, but a half dozen obvious ones won’t.
OpenOffice is a free open source office suite similar to Microsoft Office. I use it on my laptop at home. It has a word processor similar to Word, a spreadsheet tool like Excel and a presentation program similar to Powerpoint. It’s a great office suite if you don’t have have Microsoft Office. You can download OpenOffice at http://www.openoffice.org/.