Ben - In lieu of an online service, maybe some type of script or program that would compare against a dictionary of words that is stored on the site server.
Or use client-side script to utilize some popular spellcheckers that may be on the users machine already. Here is a sample of how that is done
http://www.alecmcnamara.freeserve.co.uk/spell/
Gest a cupel of thots i had abowt udder alturnatifs.