Users on thinkerlog*, BrainStorm's blog, have discovered these spell checkers.
http://www.megspace.com/computers/tinyspell/ - It adds a simple spell check to the whole system.
http://www.alcodasoftware.com - Spell Magic. Spell check each entry as you work, using a function key, or dump out the whole document as a text file and spell check it in one go. Note: it does need to be configured to use Compatibility Mode.
*Here's the link: http://brainstormsw.typepad.com/thinkerlog/2005/01/need_a_spellche.html