Apple has released Mac OS X 10.6.3, a major system update, with dozens of security fixes. This update, which patches 93 different bugs, covers the full breadth of the operating system, from AppKit to xar, by way of QuickTime, Disk Images, CoreAudio, Mail, SMB, FTP and much more.
This update is a whopping 439 MB, and is available via Software Update. As of this writing, it is not yet posted on Apple's Downloads page, but will be there soon, in both standard and combo forms.
You can get details of the security fixes here.