Moodle 2.3.3, 2.2.6 and 2.1.9 are available via the usual open download channels (http://download.moodle.org, CVS or Git).
In addition to a number of bug fixes and small improvements, security vulnerabilities have been discovered and fixed. As always, we recommend that you upgrade your sites to these latest versions as soon as possible. All admins of registered Moodle sites have been notified with security issue details.
Upgrading should be very straightforward. Full details about the releases can be found in the release notes:
Note that the 2.1 and 1.9 branches are now supported for security fixes only.
Also note that no release has been made in the 1.9 branch but there have been serious security issues fixed in this branch.