Grammatical errors are an all-too-common occurrence, but mistakes like these can determine whether or not you get a job or a high distinction on your paper. Grammarly is a web app that scrutinises your sentence structure to find errors and help you correct them.
Grammarly checks any text for free and lets you know what the errors are, but you'll need an account to get the specifics. It's not cheap at $US30 per month, but you can try it out for free for seven days, so you can at least check a resume and cover letter.
In my tests, Grammarly does a decent job of pointing out potential issues. It errs on the aggressive side, but you can always ignore its suggestions if you don't agree.