YChat's main development discussion takes place at the forums on http://yamake.5gigs.com, but that site seems to have rather unpredictable outages.
To download the latest YChat sources, you must have CVS installed on your computer. If you do, simply execute the following commands:
$ cvs -d:pserver:anonymous@cvs.sourceforge.net:/cvsroot/ychat-irc login
$ cvs -z3 -d:pserver:anonymous@cvs.sourceforge.net:/cvsroot/ychat-irc co -P ychatnew
Alternatively, you can browse the CVS repository with ViewCVS at http://cvs.sourceforge.net/viewcvs.py/ychat-irc.
Individual developers' Git repositories can be accessed via a friendly web interface from http://ychat-irc.sourceforge.net/cgi-bin/gitweb.cgi. To get the source from there, a simple
$ git clone http://ychat-irc.sourceforge.net/git/<developer>.git
should suffice.
If you are a YChat-IRC developer, then you may clone the repository via SSH by
$ git clone shell.sourceforge.net:/home/groups/y/yc/ychat-irc/htdocs/git/<developer>.git