You can use git and git-svn to work with a project's source code now. These read-only git repositories are mirrored after each commit, so they should always be in sync. The WebKit project has a wiki page which explains how to use git and svn and is a good example to follow. The URLs are of the form:

git://git.macosforge.org/<project>.git

Some projects have specialized URLs:

MacPorts
git://git.macports.org/macports/trunk.git
git://git.macports.org/macports/contrib.git
git://git.macports.org/macports/users.git

MacRuby
git://git.macruby.org/macruby/MacRuby.git
git://git.macruby.org/macruby/MacRubyWebsite.git

CalendarServer
git://git.calendarserver.org/calendarserver/CalDAVClientLibrary.git
git://git.calendarserver.org/calendarserver/CalDAVTester.git
git://git.calendarserver.org/calendarserver/CalendarClientSimulator.git
git://git.calendarserver.org/calendarserver/CalendarServer.git
git://git.calendarserver.org/calendarserver/PyAppleAuth.git
git://git.calendarserver.org/calendarserver/PyKerberos.git
git://git.calendarserver.org/calendarserver/PyOpenDirectory.git