It's built in. If you're using firefox. click the Tools button at the top, click options at the bottom of that drop down list, click the advanced tab, and make sure that the box is selected that says "Check my spelling as I type."
Then open a grateful reply to this message

and deliberately misspell a word, as I'm sure we don't spell things wrong by accident.
If you're not using Firefox, you have to download and install it first.