There was a great article yesterday that Michelle Turner pointed out on Facebook. This 12 Deadly Grammatical Errors Startups Must Avoid blog gives some of the most common simple errors. My favorites are “its or it’s” (1) “you’re or your” (6) and “flush out an idea” (9).
It’s painful to see brilliant people create blog posts with grammar so confusing that you have to work at reading the post to bring out the meaning. It’s worthwhile when you reach the end, but it would be so much more effective if one could easily read the content.
I also take issue with his “this does not apply to non-native English speakers.” My experience has been that people who have learned English as a second language are better at English usage, as they pay more attention to the rules. My advice to non-native speakers is to keep using our language and feel free to point out errors made by native speakers, as long as you do it in a non-judgmental way.