Apple just released their latest update to Leopard. 10.5.3 should appear in your software update soon. Apple states that “The Mac OS X 10.5.3 Update is recommended for Mac OS X 10.5, 10.5.1, and 10.5.2 Leopard. 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. Mac OS X 10.5 Leopard is packed with over 300 new features, installs easily, and works with the software and accessories you already have.” As of this writing the update did not appear in my software update so give it time. You can also download it standalone through Apple’s website.
Source: Macrumors.com

Update
Here is a link to the page that has the link for the file to download. Click Here! This is a large file 500MB+ so use a download manager.