It is totally free to download and this new version 2.0 comes with a built in spell check!!
I love that!
when typing posts or whatever the spelling mistakes will be underlined in red and all you do is right click- a list will pop up with options and any similar words so click on a choice and viola it's all fixed -- LOVE IT!!
{used it 3x just in this post}
http://www.mozilla.com/en-US/