Peter Norvig's spell corrector for Common Lisp.
The article How to Write a Spelling Corrector mentions Mikael Jansson and his implementation on GitHub.
This project is based heavily on that code but packages it as an ASDF installable library with the hope of getting it into the Quicklisp repository.
Example
CL-USER> (ql:quickload "spellcheck")
...
("spellcheck")
CL-USER> (spellcheck:initialize)
#<HASH-TABLE :TEST EQUAL :COUNT 29157 {100F1D7233}>
CL-USER> (spellcheck:correct "supposidly")
"supposedly"
Rob Blackwell
July 2013