The only thing I've written since high school has been shopping lists, so 'contributing' (arguable, I know) to this forum has improved my spelling. I use these for help:
http://www.thefreedictionary.com/a
http://www.askoxford.com/?view=uk
While I'm at it......we are very lucky that 99.99% of contributions to this forum are constructive and sincere. I've been on a musicians forum recently, and the childish cr@p that goes on over there has to be seen to be believed. It reminded why I got out of music.