Apple released their latest update to Leopard today - available through the usual 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
