Thee - spellcheckers are mostly a function of your browser rather than the forum you are posting in.

I use Mozilla Firefox for my main browser and an add-on spellchecker - you can even select Aussie rather than American English.
Here are some other notes about spellcheckers that were compiled for the technical section of another forum - hope they assist you
If you are using Internet explorer, there is a handy add on spell
checker you can download for free.
http://www.ie7pro.com/download.php
so when you are posting on the forum, it will underline words in red
that it thinks are miss spelled. You can right click, and select from a
list of similar words to fix. Also add words to the
dictionary.
another one is ie spell:
http://www.iespell.com/
which doesn't underline, but when you right click, and select "check
spelling", it will systematically go through everything, picking up
word by word (including the subject title). You can run both add ons,
but the red underline (ie7pro) I find the best as its simple and easy