http://www.mozilla.com/en-US/firefox/
And if something is misspelled, you will notice a little red line under the word. You can right click on the misspelled word, and the application will give you the correct spelling or choices of correctly spelled words to choose from. Try it!
Another way to improve your posts is to compose your posts in Microsoft Word, and then copy and paste them into the forum. With Word you will find similar features plus a thesaurus (shift + F7) and a grammar checker (F7) to help you express yourself in the clearest fashion.

But beware! Don't depend solely on a spell checker. Those spell checkers can be tricky!
Here's a little poem about computer spell checking:
Eye halve a spelling chequer
It came with my pea sea
It plainly marques four my revue
Miss steaks eye kin knot sea.
Eye strike a key and type a word
And weight four it two say
Weather eye am wrong oar write
It shows me strait a weigh.
As soon as a mist ache is maid
It nose bee fore two long
And eye can put the error rite
Its rare lea ever wrong.
Eye have run this poem threw it
I am shore your pleased two no
Its letter perfect awl the weigh
My chequer tolled me sew.
