I have been tweaking some of the Drupal -powered sites used at the Teaching & Learning Centre, and in the process have updated the copy of TinyMCE that provides the WYSIWYG editor. While downloading the fresh library, I noticed a spellchecker plugin , so I grabbed a copy of that while I was there.
It seems to work great! It provides a new 
button in the editor, and clicking that triggers a call to the Google spelling service . Any mistakes are highlighted automatically and alternate spellings for these are provided by clicking on the highlighted word(s).
Very handy stuff. I've enabled it on some of our sites already, and will be rolling it out for the rest of them ASAP. Here's a screenshot of it in action:
