We are developing a spell checker that we hope to finish soner rather than later. It is not going to make it into 5.0, but hopefully into a future release.
Check it out at this address: http://oacs4.polyxena.net/spellchecker/
Malte's plans for a spell checker were to make it an AOLserver module, I believe, while our spell checker is strictly meant for OpenACS.
/Ola
