Copyblogger points out 5 common gramatical mistakes that make you look dumb:

1. Your vs. You’re
2. It’s vs. Its
3. There vs. Their
4. Affect vs. Effect
5. The Dangling Participle

While we all hope that what we have to say is more important than some silly grammatical error, the truth is that some people will not subscribe, buy, or link to you if you make dumb mistakes when you write.

I agree, bad grammar makes you look dumb