The PHP developers have released another update to the 5.2 series, addressing several bugs and some security patches. In all, there were over 40 fixes included in this release.
For users upgrading from PHP 5.0 and PHP 5.1, an upgrade guide is available here, detailing the changes between those releases and PHP 5.2.13.
For a full list of changes in PHP 5.2.13, see the ChangeLog.