Anyway, to get to my point, here are the most common (and maddening) errors I see:
Weigh versus weight or weighed - I weigh x amount, NOT I weight x amount. For example, people will write something like this way too often: "Hi, I'm so-and-so and I weight xxx lbs." NO! NO! NO! YOU DO NOT WEIGHT XXX LBS! YOU WEIGH XXX LBS! Of course, you may alternately use the word "weight" by saying something like "my weight is xxx lbs." It's also necessary to mention that "weighed" is the past tense of "weigh", so "I weight myself today" is so completely wrong and will make you sound super illiterate (once again, this applies to native speakers only). What you should have typed was, "I weighed myself today".
Lose versus Loose - lose means you lost something; loose means something is too big. So, you can "lose" weight; you cannot "loose" weight. Your pants can be too "loose", however, they cannot be too "lose". Get it? Please???
These drive me nuts like the other commonly mistaken words: it's versus its, there versus their versus they're, your versus you're...UGH!!! (Now hopefully I didn't make a laughable mistake during my holier than though sounding rant, because that would just figure, wouldn't it?)