If you want the most up-to-date code, that has yet to be released (bugs and all!), you can download it via the CVS server on SourceForge. You will need a CVS client (I personally use Tortoise CVS), then enter the following information (depending on your choice of CVS client).
:pserver:anonymous@cvs.cull.sourceforge.net:/cvsroot/CULL
-d:pserver:anonymous@cvs.cull.sourceforge.net:/cvsroot/CULL
co CULL
And now you will have the most recent source. Remember that this is unreleased source, and chances are does contain a few minor niggles or more. The code you download off the CVS server must be used at your own risk
If you want an easier option, you can browse the CVS repository instead of downloading the entire source.