Have you tried Joomla's Extension Directory? (http://www.extensions.joomla.org) Other than that, you'll have to create your own custom Template ... Joomla have a tutorial on how to do that here: http://docs.joomla.org/Tutorial:Creating_a_basic_Joomla!_template
In theory, you can. But it is not recommended to make a such complicated extension that is similar to Google Code or sourceforge. If there is one, it should eat a lot of server resource since Joomla also eat a huge piece already. Sometimes, developing such complicated extension for Joomla makes you hit the wall soon. Joomla is easier to start but difficult to go too far. If you still want something similar, convert your joomla to drupal, the make a drupal module which is similar to sourceforge. Drupal is more flexible and more extensible than Joomla.