Their, there, they're.
To, too, two.
We're were, where, wear.
I'm also going to keep this short to reduce the chances of flaming. Within reason, I agree. Slack grammar and poor spelling places the burden on the reader rather than the person who wants to be read. However, it can be taken too far. The rule on not ending a sentence with a preposition “is the sort of English up with which I will not put.”
Less Churchillian, but still a favourite: Grammar is the difference between knowing your sh*t and knowing you're sh*t.