It's vs. Its

This one is a little bit special because I make this mistake so often. Since adding an apostrophe s to nouns usually shows possession, with it's, adding an apostrophe just means it is, and no apostrophe shows possession. I always forget this, and I know that many people also do as well.

Here are some examples:

It's cold outside. 
     In other words, It is cold outside.

That has its own name.
     Shows possession of name.