Download from Kernel.org
The latest version of SYSLINUX can be downloaded in .tar.gz, .tar.bz2, and .zip formats from kernel.org. This download includes both the source and official pre-compiled binaries that should work for most users (See also Official Binaries). Version changes are available in the .LSM files. Freshmeat has a good page that tracks syslinux.
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.
As of 2010-10-19, Ubuntu 10.10 (Maverick Meerkat) uses Syslinux 4.01 with GFXBoot.c32 (now officially included) and includes several special patches to allow gfxboot to use some configuration directives that were originally intented to be used only with the simple menu modules (menu.c32/vesamenu.c32). One of these patches will be included in 4.03 but the others are currently still under review.
Information how to download from the Version Control System is in the Development page.