Latest update for Mac OS X Leopard. 10.5.3 fixes plenty of bugs and weighs in at a whopping 536 megabytes for the Combo download (recommended).
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
Bug fixes nearly everywhere including Mail, iCal, Airport, Address Book, Spaces, Time Machine, and Parental Controls. MacUpdate has a nice list.