Following extensive testing over the last few weeks, Apple has finally released the first update to Mac OS X 10.5 Leopard.
The mighty 110MB update is recommended for all users of Mac OS X 10.5 Leopard.
The 10.5.1 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.
Apple offers detailed notes about the update in this support document.