Since TeXmacs-1.0.7.1, the development of TeXmacs is done using the Subversion concurrent versioning system.
Browse the TeXmacs sources on the web and follow the recent changes.
Download the most recent, but unofficial development version of TeXmacs.
Easily submit patches to TeXmacs.
Directly make changes in certain parts of TeXmacs.
In order to download the current SVN version of TeXmacs in read-only mode, you should do
svn co svn://svn.savannah.gnu.org/texmacs/trunk/src
In a similar way, you may check out the TeXmacs web site
svn co svn://svn.savannah.gnu.org/texmacs/trunk/doc
and some miscellaneous files:
svn co svn://svn.savannah.gnu.org/texmacs/trunk/misc
In order to commit changes by SVN, you first need a user account on Savannah and send a request to join the TeXmacs developers team. You may then check out and commit changes as described on the subversion page for TeXmacs.