So long awaited latest (alpha) version of Joomla! 3.0 was released yesterday. The stable version is planned for release on September 24th 2012.
Joomla 3.0 Alpha 1 is based on the new Joomla Platform versjon 12.2. However, this release contains no new features. New features will be added in next test version in Alpha 2 (~July 23rd) and possibly also in the first beta version due in late August.
Which features ultimately will go to the newest release is not entirely certain. This is dependent on quality, and thus whether they will be completed and corrected in time. The CMS Feature Tracker has the list of proposed features. While the front and admin end of the new version are much improved for end users (new user-interface enhancements), developers will find a variety of important updates as well.
Click on this link: Joomla 3.0 Alpha 1 for download.
p.s.
Joomla! 2.5 is a long-term-support (LTS) release and will be supported at least until September 2013.