I have this base64 implementation (both decoding and encoding) in Common Lisp available on my web page. I have had some very nice feedback about it, but I've been a lazy maintainer; KMR sent me some patches for ACL compatibility many moons ago but I never got around to integrating them. Um. I think. Or maybe I did... Well, anyway, it took ages.
Now it turns out that a) Edi Weitz created a page for it in CLiki and b) KMR forked base64, now calling it cl-base64, giving it some undoubtedly much-needed optimizations (I never benchmarked my implementation and speed wasn't much of a concern when coding it) and adding some nice extra functionality.
Thanks, guys, both of you.