The 10.5.7 Update is recommended for Mac OS X 10.5 Leopard and includes general operating system fixes that enhance the stability, compatibility and security of your Mac. For detailed information about security updates, please visit this website.
I notice a lot of sync-related "reliability" fixes here, some nice additions to the parental controls, a Gmail login fix, and widget updates (yeah, some people use those). Full details here. Enjoy!