Along with the 10.6.2 update, Apple released a Security Update for 10.5. This Update is recommended for all users and improves the security of Mac OS X. Previous security updates have been incorporated into this security update, so if you’ve skipped some updates, this one will catch you up quickly.
If you have 10.5, you can download the update by running Software Update, which is found under the Apple button in the upper left corner of your screen (this is the recommended method). You can manually install the update by clicking here.