Joomla! 2.5 is the current LTS version. The previous LTS version, Joomla 1.5, is still widely used and will be supported with security fixes only until September 2012.
For more information see the Joomla 2.5 category page.
Joomla 3.3 is the latest STS version after Joomla 2.5. Support for Joomla 3.3 is still active.
For more information see the Joomla 3.3 category page.