Nightly builds are available of the Joomla project which include all of the latest additions and fixes. anyone can easily download it and install it on any Joomla instance to upgrade it to the latest build version.
Check out the video overview of where you can download the latest build and how you can install it on your website.
If there is a fix that has been created and tested for the next release of Joomla but you need it right now, you can download the nightly build and install that on to your website. It will have all of the documented features and issues that have been fixed and added in for the next release, but beware, it may also have other issues that people aren't aware of yet.
Of course there is a large amount of testing that is done on these patches and fixes but not everything is picked up so you've been warned. This is something for the more experienced Joomla developers and site builders.
You can download the nightly builds at: http://developer.joomla.org/cms-packages/