Running two Trac instances at once, one of which is pointing to a single project only. This way you can password protect all your projects but one. See blog post for details: http://tmblr.co/ZRgRYwfFmZiH.

gistfile1.apacheconf

# Private, password protected instance for all projects
<Location /trac>
        SetHandler mod_python
        PythonInterpreter main_interpreter
        PythonHandler trac.web.modpython_frontend
        PythonOption PYTHON_EGG_CACHE /tmp/python_egg_cache
        PythonOption TracEnvParentDir /home/services/trac/projects

        AuthType Basic
        AuthName "innovationTM authentication"
        AuthUserFile /home/services/htpasswd
        Require valid-user
</Location>

# Public, non password protected instance for single project, 'newproject'.
<Location /newproject_pub>
  SetHandler mod_python
  PythonInterpreter main_interpreter
  PythonHandler trac.web.modpython_frontend
  PythonOption TracEnv /home/services/trac/projects/newproject
  PythonOption TracUriRoot /newproject_pub

  # Uncomment lines below (and create valid htpasswd.newproject.pub file) 
  # to protect 'newproject' Trac with different set of passwords than 
  # other projects.

  #AuthType Basic
  #AuthName "newproject_pub"
  #AuthUserFile /home/services/htpasswd.newproject.pub
  #Require valid-user
</Location>