Debian 6.0, squeeze, has been the stable version of Debian for over 18 months now. With the relatively slow release cycle for Debian distros, it will likely be the current release for at least another year, and because of this it is of course getting point updates. Debian 6.0.6 is available now, as part of the ongoing support.
The update includes up-to-date packages compared to the previous ISOs, as well as various security updates. Packages that have been updated include nVidia drivers, Plymouth, Memcached, and the Linux Kernel. Security fixes are applied to Iceweasel, PHP 5, OpenOffice.org, qemu, and many more.
If you’re already running Debian 6.0, a simple apt-get update will keep you up to date with these changes. You can grab the ISO from the Debian website.