Apple released its latest update to Leopard after about a month from the last update. 10.5.4 fixes issues in iCal, Safari, Airport, Spaces, and Exposé as well as others.

The Mac OS X 10.5.4 Update is recommended for Mac OS X 10.5, 10.5.1, 10.5.2, and 10.5.3 Leopard. It includes general operating system improvements that enhance the stability, compatibility, and security of your Mac.

Source: Apple.com