Official Testing versions (aka pre-release), when available, can be downloaded from:
The latest official version of Syslinux can be downloaded in .tar.gz, .tar.bz2, .tar.xz, 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.
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 intended to be used only with the Simple Menu modules (menu.c32/vesamenu.c32). Some patches are upstreamed, but some of them might not.
Information about how to download from the Version Control System is in the Development page.