The latest version of SYSLINUX can be downloaded in .tar.gz, .tar.bz2, and .zip formats from kernel.org .LSM files will give you the changes from one version to the next.
The SYSLINUX download includes PXELINUX, ISOLINUX and MEMDISK as well.
Freshmeat has a good page that tracks syslinux.
WARNING - at least SuSE, Mandriva, and Ubuntu use a version of SYSLINUX modified with a patch called "gfxboot". This is a highly invasive and unsupported modification of SYSLINUX. Please avoid these versions if possible.