Interesting UI: Each spelling error is a hyperlink and when selected the options are listed in another box. When one of those options are selected, that word is replaced in the text.
XSpell is a spell checker for HTML forms. It is composed of a XML-RPC based browser client written in Javascript (requires IE 5.x since it's based on DOM) and an XML-RPC service implemented in PHP 4+. Both of these components are OpenSource.
Home page blurb: Speller Pages is a Web interface and JavaScript library for a server-side spell check program. Available in PHP or Perl/CGI versions, Speller Pages allows you to bring the superior spelling suggestion capabilites of GNU Aspell to users of your Web site.