One collection of DES related links is at http://www.tcs.hut.fi/~helger/crypto/link/block/des.html

This collection includes pointers to the algorithms used by the d.net client (BrydDES and Kwan). One bitslicer not included in that list is from Andrew Meggs - the source to that in the d.net client source ( http://www.distributed.net/source/)

DES implementations are also often available in the many operating systems that come in source. For example OpenBSD http://www.openbsd.org/ and FreeBSD http://www.freebsd.org/. Solaris has made its 'secure download' source available too at http://www.sun.com/software/solaris/encryption/download.html

A bibliography on DES (and other block-cypher related) material is at http://www.ii.uib.no/~larsr/bc.html

For a general overview on crypto, I suggest you try and get the book "Applied Cryptography" by Bruce Schneier. You may also wish to review http://www.tcs.hut.fi/~helger/crypto/