Apple released the Mac OS X 10.5.3 update today. The latest update to Leopard should appear in your Mac OS X Software Update.

The 10.5.3 Update is recommended for all users running Mac OS X Leopard and includes general operating system fixes that enhance the stability, compatibility and security of your Mac.

For detailed information on this update, please visit this website: http://support.apple.com/kb/HT1141.
For detailed information on security updates, please visit this website: http://support.apple.com/kb/HT1222.
Apple details the many fixes in their support document. Some highlights include:

- Includes additional RAW image support for several cameras
- Addresses an issue with stuttering video and audio playback in certain USB devices
- Improves 802.1X (Wifi) behavior and reliability and when using Time Machine
- Includes fixes for Time Machine compatibility with Time Capsule