I gotta disagree. I think that "there", "their", "and they're" is the most common mistake. And it drives me nuts.
The it's/its thing doesn't bother me too much because I think it's sometwhat reasonable for people to think that "its" should have an apostrophe because possessives (other than possessive pronouns) always have apostrophes.
I must agree here.
I recently learned it is indeed allowable to end a sentence with a preposition. That was so freeing. Though I do believe ending a sentence with a proposition can be more entertaining.