WillyP is immune to irony. Or so he says...
Come on people! You’re on a computer! Nearly everything has a spell checker! Nearly all word processors have a decent grammar checker as well. Simple and quick to use! A lot of web browsers even provide simple spell checking (Firefox, Chrome), all you have to do is know enough to use the right word! Even then, if in doubt, Google it!
I want to mention very quickly that some people don't speak english natively and don't have the time or the energy to learn it perfectly. Some people don't have the time to learn it at all and have to resort to translators which are by their nature imperfect. Don't be so quick to judge, especially on the internet where there's no way to know everything that's going on.
What it really boils down to is how professional and mature you want to be about many of these errors. And by "professional" and "mature," I certainly don't mean by being a grammar nazi (a term which actually contains an error itself, BTW. "Nazi" is a proper noun and should always be capitalized). Mistakes are still going to happen, and with such a large group of people on the Internet at any given time, odds are there's going to be a lot of them, so one should be more understanding and forgiving in most cases than annoyed.