A Common Lisp implementation of ARC4, a Cryptography code, can be found on the Ciphersaber page or here. Note that experts in sci.crypt do not believe that the key management proposed there is safe.

Another implementation may be found here, which discards the first 3072 bytes of the crypto-stream as recommended. It also provides a general-purpose seedable PRNG (pseudo-random number generator) as part of the API (inherently used as part of any stream-cipher implementation but not always exported).

Ironclad also includes an implementation.