The Spelling Checker plug-in for WordPress provides a built-in facility for
spelling checks on posts from within the administration pages as well as
spelling checks on comments via simple calls added to your templates. It
requires no changes to the WordPress code-base to use, you merely drop it in to
the plug-ins folder and enable it in the interface. This version has been tested
in both WordPress 1.2.1 as well as WordPress 1.5 Strayhorn.
Free Download: Download this wp-plugin