Mac OS X 10.5.6 Combo Update is Apple's recommended update for all users running Mac OS X Leopard 10.5.0.
Mac OS X 10.5.0 includes general operating system fixes that enhance the stability, compatibility and security of your Mac. It is recommended for Mac OS X 10.5 Leopard versions 10.5, 10.5.1, 10.5.2, 10.5.3, 10.5.4 and 10.5.5. There are many improvements and fixes in this update including to Address Book, AirPort, Client management, iChat, Graphics, Mail, MobileMe, Networking, Printing, Parental Controls, Time Machine, Safari and other generaL fixes and improvements. Full details can be found here.
Mac OS X 10.5.6 Combo Update is an essential download for Leopard users.