cocoAspell is Mac OS X interface to Aspell — A more intelligent Ispell — that is being developed by Kevin Atkinson. Here is a brief snippet of how Kevin describes Aspell on his web site:
Aspell is an Open Source spell checker designed to eventually replace Ispell. Its main feature is that it does a much better job of coming up with possible suggestions than Ispell does. In fact recent tests shows that it even does better than Microsoft Word 97’s spell checker or just about any other spell checker I have seen. It also has support for checking (La)TeX and HTML files, and run time support for other non English languages.