Start using your one-click update fingers: Joomla 2.5.4 has been released.
This release is all about bug fixes. Unlike Joomla 2.5.3, there aren't any major security problems fixed in this release. Joomla 2.5.3 was a very rare exception and fixed the only major security hole found in three years.
Unless you have Joomla 1.5, there's now only one correct answer to the question "what version of Joomla are you running?" The answer is Joomla 2.5.4.
You should upgrade to Joomla 2.5.3 from any previous version of Joomla 1.6, 1.7 or 2.5.
We've got full instructions on how to upgrade here.
If you are on Joomla 1.5, don't worry because your version doesn't require an update. There's no rush, but if you do eventually want to upgrade to Joomla 2.5, here are two useful tutorials: