Apple released the Mac OS X 10.5.5 update, which is recommended for Mac OS X Leopard versions 10.5, 10.5.1, 10.5.2, 10.5.3 and 10.5.4.
The new OS X update contains general operating system improvements that enhance the stability, compatibility, and security of your Mac.

OS X 10.5.5 rolls up recent security updates and fixes some weird bugs like some Macs unexpectedly powering on at the same time each day.
Besides several general fixes the OS X 10.5.5 update touches address book, Disk Utility and Directory Utility, iCal, Mail, Mobile Me, and Time Machine.
The last update (OS X 10.5.4) was released on July 1st.
More details in the OS X 10.5.5 release notes on the Apple site.