Although grammar, spelling and punctuation appears to be optional these days, particularly on the web, it definitely doesn't hurt to at least know the rules of the English language. At best, it might help you find a better job. At worst, you can always hold the grammar card in your back pocket, in life-threatening situations (yeah, he might have a new truck, but does he know the difference between your and you're?).
The Online Writing Lab at Purdue has a page of Interactive Exercises that can help with some of the most common problem areas. There are tests pertaining to adjectives/adverbs, apostrophes and spelling. In my opinion, the rules regarding "effect" and "affect" are the most difficult. As a secret, I remember the term "neva," because usually, "effect" is a noun, while "affect" is a verb. Then again, I scored a 60 percent on the test, so maybe that isn't completely effective.
Another item that can sometimes be confusing is distinguishing between the usage of "good" and "well." There's a test for that as well. And yes, that was the correct use for that word. Yep, I done good! Whoa, that's bad ... it looks like I need to retake the tests immediately.