Are you using Firefox? Have you get the Firefox spell checker enabled?

This checks your spelling as you type so you can correct it before posting.

It underlines words that may be misspelt. You then right click the underlined word and some suggestions are shown in a menu and you can left click on one.

On the Firefox tool bar at the top left of the page , 6th from left is: tools

Click on Tools.
Click on Options (found at the bottom).
Click on Advanced (found at the top right).
General tab.
Underneath 'Browsing';
Tick 'Check my spelling as I type'.
Click OK.

It's not perfect, but it should help.

To improve it further there is a British English dictionary you can install as a Firefox 'add on'.

https://addons.mozilla.org/en-US/firefox/addon/3366
Or https://addons.mozilla.org/en-US/fir...y-/?src=search
Or https://addons.mozilla.org/en-US/fir...-2/?src=search

I see from the reviews that a lot of people are having compatibility problems with this now, however works fine for me using Firefox 3.5.3 to 25.0.