Grammar checkers? Aaargh! Run for the hills! They are evil. Evil, I tells ya! (Green wiggly line there...)
Someone well-meaning told me about this one, though:
http://www.whitesmoke.com
Apparently it analyses text and it's utterly brilliant, or so it said in the Observer last Sunday.
I haven't checked it out yet, such is my extreme sceptism.
<Added>Spellcheckers can be quite useful, though.
Scepticism. Sigh. Kind of ruins my point, doesn't it? ;)