There's always the Google toolbar but it will follow all your tracks. I just use my basic word processor's spell check, then copy 'n paste. When it can't figure out what word I intended, I'll consult G and get "Did you mean..." I find it's usually pretty accurate plus the dictionary is a click away if it doesn't look quite right to me.

A thought/punctuation/syntax/~participle checker is what I need. It's just the web though... No one is going to smack your knuckles with a ruler or anything.