Apple has released the OS X 10.5.5 update, available now via Software Update. From their update information page:
The Mac OS X 10.5.5 Update is recommended for Mac OS X Leopard versions 10.5, 10.5.1, 10.5.2, 10.5.3 and 10.5.4. It includes general operating system improvements that enhance the stability, compatibility, and security of your Mac.
For detailed information about security updates, please visit this website.
You can get the standalone installer here.